UK Solar System Data Centre

The time and scale size of planetary wave signatures (PWS) in the mid latitude F region ionosphere of the Northern Hemisphere and the main pattern of their possible sources of origin are presented. The PWS involved in this study have periods of about 2-3, 5-6, 10, 13.5, and 16 days. The PWS in the ionosphere are large scale phenomena. PWS with periods of about 2-3 and 5-6 days have a typical longitudinal size of 80^o, they are coherent some 6000 km apart, and they occur about 12% and 14% of the entire observational record respectively. The typical longitudinal size of PWS with periods of about 10 and 13 days is 100^o, they are coherent some 7500 km apart, and they occur about 24% and 22% of the entire observational record respectively. PWS with periods of about 16 days seem to be global scale phenomena, and they occur about 30% of the entire observational record. The results estimate that geomagnetic activity variations play the most important role for driving PWS in the ionosphere. The geomagnetic activity variations can drive at least 20-30% of the PWS with periods of about 2-3, 5-6, 10 and 16 days, but even up to 65-70% for the PWS with periods of about 10 and 16 days, and they practically drive 100% of the PWS with periods of about 13.5 days. The planetary wave activity in the mesosphere/lower thermosphere (MLT) winds can drive about 20-30% of the PWS with periods of about 2-3, 5-6, 10 and 16 days. There is a significant percentage of existence of PWS in the F region apparently 'independent' from the geomagnetic activity variations and of the MLT winds. The latter is better expressed for PWS with shorter period. PWS with periods of about 13.5 days are an exception to that. A candidate mechanism for the 'independent' events may be the non linear interaction or the amplitude modulation between different PWS.

Analyses of time-spatial variations of critical plasma frequency foF2 during the summer of 1998 reveal the existence of an oscillation activity with attributes of a 6-day westward propagating wave. This event manifests itself as a global scale wave in the foF2 of the Northern Hemisphere, having a zonal wave number 2. This event coincides with a 6-day oscillation activity in the meridional neutral winds of the mesosphere/lower thermosphere (MLT). The oscillation in neutral winds seems to be linked to the 6-7-day global scale unstable mode westward propagating wave number I in the MLT. The forcing mechanisms of the 6-day wave event in the ionosphere from the wave activity in the MLT are discussed.

The advantage of studying eclipse disturbances is the perfect predictability of their 4D source geometry, which allows for preparation of adapted systems and schedules. The total solar eclipse period of August 11, 1999 across Europe was notable for exceptionally uniform solar disk, steady solar wind and quiet magnetospheric conditions. Large-scale gravity wave activity prior to the eclipse however disturbed the initial 0900 LT thermosphere weather. This rapid letter is an advance summary about one particular aspect of the West European ionosonde and radar results of the eclipse experiment. It focusses on the possible emergence of a distant eclipse frontal bow-wave. This was expected as a consequence of the supersonic shock of stratospheric Ozone cooling. First-look data of Vertical Incidence Digisonde records are greatly improved by their Real-Time acquisition of inverted true-height profiles. The EBRE (Tortosa, Spain) foF1 and foF2 simultaneous oscillations observed from the second to the fourth hour following maximum solar occultation appear as convincing indicators of the bow-wave signature. Large fluctuations in foF1 and foF2 during some of our control days, of usual gravity wave character, emphasize the importance of meteorologic disturbances on mid-latitude ionosphere variability.

Traveling Ionospheric Disturbances (TIDs) are wave-like propagating irregularities that alter the electron density environment and play an important role spreading radio signals propagating through the ionosphere. A method combining spectral analysis and cross-correlation is applied to time series of ionospheric characteristics (i.e., MUF(3000)F2 or foF2) using data of the networks of ionosondes in Europe and South Africa to estimate the period, amplitude, velocity and direction of propagation of TIDs. The method is verified using synthetic data and is validated through comparison of TID detection results made with independent observational techniques. The method provides near real time capability of detection and tracking of Large-Scale TIDs (LSTIDs), usually associated with auroral activity.

Keywords: Travelling Ionospheric Disturbances (TIDs), real-time specification and tracking of Large Scale TIDs (LSTIDs), ionosphere, auroral activity

Multilayer perceptron type neural networks (NN) are employed for forecasting ionospheric critical frequency (foF2) one hour in advance. The nonlinear black-box modeling approach in system identification is used. The main contributions: 1. A flexible and easily accessible training database capable of handling extensive physical data is prepared, 2. Novel NN design and experimentation software is developed, 3. A training strategy is adopted in order to significantly enhance the generalization or extrapolation ability of NNs, 4. A method is developed for determining the relative significances (RS) of NN inputs in terms of mapping capability.

Solar cycle variations of the amplitudes of the 27-day solar rotation period reflected in the geomagnetic activity index Ap, solar radio flux F10.7cm and critical frequency foF2 for mid-latitude ionosonde station Moscow from the maximum of sunspot cycle 18 to the maximum of cycle 23 are examined. The analysis shows that there are distinct enhancements of the 27-day amplitudes for foF2 and Ap in the late declining phase of each solar cycle while the amplitudes for F10.7cm decrease gradually, and the foF2 and A_p amplitude peaks are much larger for even-numbered solar cycles than for the odd ones. Additionally, we found the same even-high and odd-low pattern of foF2 for other mid-latitude ionosonde stations in Northern and Southern Hemispheres. This property suggests that there exists a 22-year cycle in the F2-layer variability coupled with the 22-year cycle in the 27-day recurrence of geomagnetic activity.

Recent theoretical model simulations of the ionosphere response to geomagnetic storms have provided the understanding for the development of an emperical storm-time ionospheric model (STORM). The emperical model is driven by the previous time-history of a_p, and is designed to scale the quiet-time F-layer critical frequency (f_oF₂) to account for storm-time changes in the ionosphere. The model provides a useful, yet simple tool for modeling of the perturbed ionosphere. The quality of the model prediction has been evaluated by comparing with the observed ionospheric response during the Bastille Day (July 2000)storm. With a maximum negative D_st of -290 nT and an a_p of 400, this magnetic perturbation was the strongest of year 2000. For these conditions, the model output was compared with the actual ionospheric response from all available stations, providing a reasonable latitudinal and longitudinal coverage. The comparisons show that the model captures the decreases in electron density particularly well in the northern summer hemisphere. In winter, the observed ionospheric response was more variable, showing a less consistent response, imposing a more severe challenge to the emperical model. The value of the model has been quantified by comparing the root mean square error (RMSE) of the STORM predictions with the monthly mean. The results of this study illustrate that the STORM model reduces the RSME at the peak of the disturbance from 0.36 to 0.22, a significant improvement over climatology.

[1] The latest version of the International reference ionosphere, IRI2000 [Bilitza, 2001], contains a dependence on geomagnetic activity based on an empirical storm-time ionospheric correction model (STORM) [Araujo-Pradere et al., 2002]. The new storm correction in IRI is driven by the previous time history (33 hours) of a_p and is designed to scale the normal quiet-time F layer critical frequency (f_oF₂) to account for storm-time changes in the ionosphere. An extensive validation of IRI2000 has been performed during geomagnetic storm conditions to determine the validity of the new algorithms. The quality of the storm-time correction has been evaluated by comparing the model with the observed ionospheric response during all the geomagnetic storms with a_p>150 in 2000 and 2001, a total of 14 intervals. The model output was compared with the actual ionospheric response for all available ionosonde stations for each storm. The comparisons show that the model captures the decreases in electron density particularly well in summer and equinox conditions. To quantify the improvement in IRI2000, the root-mean-square error has been evaluated and compared with the previous version of IRI, which had no geomagnetic dependence. The results indicate that IRI2000 has almost a 30% improvement over IRI95 during the storm days and is able to capture more than 50% of the increase in variability, above quiet times, due to the storms.

IRI2000 [Bilitza, 2001] now contains a geomagnetic activity dependence based on the Time Empirical Ionospheric Correction Model (STORM) [Araujo-Pradere and Fuller-Rowell, 2002; Araujo-Pradere et al., 2002]. The storm correction is driven by the previous time history of a_p and is designed to scale the quiet time F layer critical frequency (f_oF₂) to account for storm-time changes in the ionosphere. The quality of the storm-time correction was recently evaluated by comparing the model with the observed ionospheric response during all the significant geomagnetic storms in 2000 and 2001. The model output was compared with the actual ionospheric response at 15 stations for each storm. These quantitative comparisons using statistical metrics showed that the model captures the decreases in electron density particularly well in summer and equinox conditions, but is not so good during winter conditions. To further assess the capabilities of the model, STORM has been compared in detail with observations during the Bastille Day storm in July 2000. This storm, considered to be on the extreme end of the statistical scale of storm magnitude, highlights two main areas were challenges remain for the empirical storm-time ionospheric model. The first is the rapid onset of the positive storm phase; the second is the regional composition changes that can affect one longitude sector at the expense of another for a particular storm. Both these challenges, although appreciated during the development of STORM, remain to be addressed.

Using data from 75 ionosonde stations and 43 storms, and based on the knowledge gained from simulations from a physically based model, we have developed an emperical ionospheric storm-time correction model. The model is designed to scale the quiettime F region critical frequency (f_oF₂) to account for storm-time changes in the ionosphere. The model is driven by a new index based on the integral of the a_p index over the previous 33 hours weighted by a filter obtained by the method of singular value decomposition. Ionospheric data was stored as a function of season and latitude and by intensity of of the storm, to obtain the corresponding dependencies. The good fit to data at midlatitudes for storms during summer and equinox enable a reliable correction, but during winter and near the equator, the model does not improve significantly on the quiet time International Reference Ionosphere predictions. This model is now included in the international recommended standard IRI2000[Bilitza 2001] as a correction factor for perturbed conditions.

[1] STORM is an empirical ionospheric correction model designed to capture the changes in F region electron density during geomagnetic storms. The model is driven by the previous 33 hours of a_p, and the output is used to scale the quiet time F region critical frequency (f_oF₂) to account for increases or decreases in electron density resulting from a storm. The model provides a simple tool for modeling the perturbed ionosphere. The quality of the model has been evaluated by comparing the predictions of the model with the observed ionospheric response during the six storms in the year 2000. The model output has been compared with the actual ionospheric response at 15 ionosonde stations for each storm. The comparisons show that the model captures the decreases in electron density particularly well in summer and equinox at midlatitudes and high latitudes but is less accurate in winter. The value of the model has been quantified by comparing the daily root mean square error of the STORM predictions with the monthly mean. The results of the validation show that there is a 33% improvement of the STORM model predictions over the monthly median during the storm days and that the model captures more than half of the increase in variability on the storm days, a significant advance over climatology.

An ionospheric F₂ critical frequency database has been assembled to determine the variability of the F region as a function of local time, latitude, season, and geomagnetic activity. The database comprises observations from 75 ionosonde stations covering a range of geomagnetic latitude and includes 43 storm intervals. The database was previously used to develop the Storm-Time Empirical Ionospheric Correction Model (STORM). The mean and standard deviation have been evaluated by sorting the data by local time, season (five intervals centered on equinox, solstice, and intermediate intervals), latitude (four regions each 20^o wide in geomagnetic latitude), and up to eight levels of geomagnetic activity. The geomagnetic activity index was based on a weighted integral of the previous 33 hours of ap and is the same as that used by the STORM model. The database covers a full solar cycle, but insufficient information was available to sort by solar activity without compromising the estimates of variability on the other sorting parameters. About half the data were contained in the first level of geomagnetic activity, between 0 and 500 units of filtered ap corresponding to K_p <= 2, and half above that level. When local time dependence was included in the binning, sufficient data were available to sort into two levels of geomagnetic activity, quiet (K_p <= 2⁺) and disturbed (K_p > 3^-). For all latitudes and levels of geomagnetic activity, the lowest variability was typically found in summer (10-15%), and the largest variability occurred in winter (15-40%), with equinox (10-30%) lying between the solstice extremes. The exception was low latitudes at equinox, which had surprising low variability (10%), possibly because of the weak interhemispheric flow at this time of year. At middle and low latitudes, the variability tended to increase with geomagnetic activity in winter and equinox but remained fairly constant in summer. At high latitudes, the surprising result was that in all seasons, and in winter in particular, the variability tended to decrease, probably because of the increased upwelling of neutral molecular species and stronger chemical control of the ionosphere. The data have also been used to build a table of estimated variability suitable for inclusion in the International Reference Ionosphere or any other climatological model. For periods where data were scarce or nonexistent, an estimated variability was provided on the basis of expectations of the consequences o physical processes. This was necessary to fill in the table o values in order to develop a module suitable for inclusion in th International Reference Ionosphere.

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Throughout the solar cycle, average nighttime thermospheric meridional winds are larger in spring than autumn despite similar levels of solar insolation. The average ion velocities are also larger in spring than autumn at solar maximum, but at solar minimum this position is reversed. Numerical simulations of the thermosphere and ionosphere have not predicted such asymmetries because they generally assume forcing functions that are symmetric about the solstices. The proposed explanation lies in the annual and diurnal variation in solar wind-magnetosphere coupling caused by changes in the orientation of the geomagnetic pole, and hence the magnetosphere, with respect to the average orientation of the IMF (the Russell-McPherron effect). This causes a 12-hour phase difference between the times of maximum solar wind-magnetosphere coupling at the two equinoxes. In addition, the orientation of the geomagnetic axis with respect to the average IMF is such that <B_y*B_z> > 0 for the March equinox and <B_y*B_z> < 0 for September. This results in a further source of asymmetry of forcing of the high-latitude ionosphere as the result of electric fields associated with the four sign combinations of B_y and B_z. Several predictions arise from the explanation given: for example, a high-latitude station measuring thermospheric neutral winds in Alaska, 180^o in longitude from Kiruna, might be expected to see nighttime thermospheric winds that are larger in the autumn than in the spring.

Fabry-Perot Interferometer measurements of neutral winds and European Incoherent SCATter radar measurements of plasma velocities have shown a significant equinoctial asymmetry in the average behavior of the thermosphere and ionosphere above northern Scandinavia. Existing standard models of the upper atmosphere use forcing functions that are symmetric about the solstices, therefore these observations are unexpected. It is suggested that the asymmetry arises from the diurnal variation in the cross polar cap potential difference (CPCPD) because there is a 12 hour phase difference between the variations at the March and September equinoxes. The variation in the CPCPD is caused by an annual and diurnal variation in the orientation of the magnetosphere with respect to the interplanetary magnetic field. This is known as the Russell-McPherron(R-M) effect. The plausibility of this explanation of the equinoctial asymmetry in thermospheric winds is supported by investigation of the effect of their geomagnetic history, i.e., the repercussions on the winds of the activity levels in the few hours prior to the observation. The consequences of the R-M effect have been simulated in the University College London/Sheffield/Space Environment Laboratory coupled thermosphere-ionosphere model by imposing a diurnally varying high-latitude electric field pattern. The results are used to test the predictions, given in an earlier paper, of the average behavior expected at other high-latitude sites. A corollary to the study is that the evidence presented here implies that the auroral oval may be smaller at solar minimum, which is also unexpected.

The Aberystwyth tomographic imaging experiment and the Sheffield Coupled Thermosphere-Ionosphere-Plasmasphere model (SCTIP) have been used to investigate the dayside ionospheric trough at high latitude under different geomagnetic conditions. Previous work has suggested that the latitude of the trough minimum and the structure of the poleward wall is dependent on the electron precipitation, whereas the formation of the trough itself is dependent on the convection of flux tubes. We further discuss the roles of flux tube convection and the nightside stagnation region in the formation of both the dayside and nightside troughs, and the role of partially depleted flux tubes in the observed equatorward structuring of the trough region.

We review substantial recent developments to the CTIP coupled thermosphere-ionosphere-plasmasphere model, using observations from the Aberystwyth ionospheric tomographic imaging chain and the IMAGE satellite to benchmark and validate the model results. Thermospheric heating in auroral regions has classically been viewed as a combination of Joule Heating (macroscopic frictional heating from the ionosphere), Lorentz forcing (microscopic momentum transfer from ions) and particle precipitation. Of these, it has been shown that above about 110 km, Joule Heating is the dominant energy transfer mechanism. However, ion velocities during disturbed times often approach or exceed the neutral sound speed. We investigate shock front heating through modelling using the improved CTIP model and compare these with in-situ satellite observation. We conclude that shock heating may be a significant contribution to the auroral thermosphere-ionosphere energy balance.

The localised "night" created as the moon's shadow travelled across the Earth during the total solar eclipse of 11th August 1999, produced changes in the ionosphere across Europe that were monitored with a variety of modern instrumentation. The passage of the 100km wide, super-sonic lunar shadow offered the opportunity to examine the changes in electron densities, radio absorption, neutral wind patterns and the possible generation of waves in the layers of the ionosphere. All these for an event for which the cause of the disturbance can be calculated with accuracy. Reported here are the results from the vertical ionosondes located under the path of totality and in the partial eclipse region and dual frequency GPS TEC measurements. The ionosondes showed that even in the partial shadow the peak electron densities of the F & E ionospheric layers decreased by as much as 20-35%. The TEC measurements showed that the vertical equivalent line integrated electron density dropped by 15% at the 97% partial eclipse north of the path of totality. The consequences of these observations are discussed in relation to making model predictions.

The main objective of DIAS (European Digital Upper Atmosphere Server) project is to develop a pan-European digital data collection on the state of the upper atmosphere, based on real-time information and historical data collections provided by most operating ionospheric stations in Europe. A DIAS system will distribute information required by various groups of users for the specification of upper atmospheric conditions over Europe suitable for nowcasting and forecasting purposes. The successful operation of the DIAS system will lead to the development of new European added-value products and services, to the effective use of observational data in operational applications and consequently to the expansion of the relevant European market.

Keywords: Ionosphere; Upper atmosphere; Ionospheric monitoring; Ionospheric nowcasting; Ionospheric forecasting; Digital libraries

Ionospheric observations from nine middle-latitude stations are studied for five magnetic storms that occurred during September and October 2000. The correlation between various solar wind, magnetospheric and ionospheric parameters shows that the nighttime ionospheric response is strongly dependent on the conditions during which solar wind-magnetosphere coupling occurred. Storms with initial compressive phase and rapidly evolving main phase have as a global effect the ionization depletion in the nightside at middle latitudes, independent of the storm intensity. These storms are caused by the abrupt dissipation of a large amount of energy input, resulting in the rapid expansion of the neutral composition disturbance zone equatorward, producing the observed negative effects in all middle latitude stations presented here. Gradually evolving geomagnetic storms, driven by slowing increasing southward IMF, result in the observation of positive effects at night in low to middle latitude stations. The weaker the intensity of the storm is, according to the Dst index, the more likely it is that one will observe nighttime ionization enhancements in subauroral latitudes as well. There are two competing mechanisms causing the observed effects; the expansion of the neutral composition disturbance zone results in negative effects, while downward plasmaspheric fluxes produce ionization enhancements at night. Gradually evolving storms are characterized by the restricted development of the neutral composition disturbance zone to higher latitudes, and the extent of its equatorward boundary depends on the intensity of the storm. During storms of this type, the role of plasmaspheric fluxes dominates at middle to low latitudes. Their effects are observable up to subauroral latitudes given that the neutral composition disturbance zone development is restricted to higher latitudes, as happens when the geomagnetic activity is of low or moderate intensity.

Independent of the possible sources (solar activity, geomagnetic activity, greenhouse effect, etc.) of a global change in the upper atmosphere, it is the sign of a long-term trend of temperature that might reveal the cause of a global change. Long-term change of temperature in the F region of the ionosphere has been studied and is assumed to be expressed in terms of thickness of the bottornside F2 layer characterized by the difference between height of the maximum electron density of the F2 layer hmF2 and altitude of the lower boundary of the F region represented by h'F. Using the difference of two ionospheric parameters has the advantage that it reduces the effect of changes resulting from alteration of equipment and scaling personnel. In this study, in summer only night values of the difference hmF2-h'F and in winter both day and night values have been taken into account considering that h'F might indicate the lower boundary of the F region in these periods. The study of the behaviour of hmF2-h'F taking separately the stations and determining yearly the mean measure (trend) of the variation of hmF2-h'F with solar and geomagnetic activities found that this difference increases significantly with enhanced solar activity, but trends of the solar activity effect exerted on this difference themselves do not practically change with increasing sunspot number. Further, hmF2-h'F decreases only insignificantly with growing geomagnetic activity. Trends of the geomagnetic activity effect related to hmF2-h'F change only insignificantly with increasing Ap; however, trends of the geomagnetic activity effect decreased with increasing latitude. As a result of this investigation it has been found that hmF2-h'F regarded as thickness of the bottornside F2 layer shows an effect of the change of solar activity during the last three solar cycles, indicating temperature change in the upper atmosphere to be expected on the basis of changing solar activity. Furthermore, though a long-term variation of solar activity considering only years around solar activity minima is relatively small, the difference hmF2-h'F indicates a trend opposing the change of solar activity; that is, it decreases slightly during the first three 20, 21, 22 solar cycle minima (1964-1986), but decreases more abruptly according to the change of solar activity towards the minimum of solar cycle 23 (1986-1996), thus also indicating variation of temperature in the F region. However, this variation cannot be explained by the change of solar and geomagnetic activities alone, but assumes some other source (e.g. greenhouse gases) too.

The results of model computations of thermospheric and ionospheric effects of the solar eclipse of August 11, 1999, are reported. The computations are performed in terms of a self-consistent global model of the Earth's thermosphere, ionosphere, and protonosphere. It is shown that during the eclipse, the neutral gas temperature in the thermosphere decreases by 90 K, absolute concentrations of O and N₂ components decrease by 20 and 40%, respectively, and the wind regime changes so that it allows the amplitude of neutral gas velocity to change by 100 m/s. The results of foF2 computations are compared to the experimental data obtained at Chilton station (51.3^oN, 1^oW) during the eclipse of August 11, 1999. The decrease in foF2 reaches similar to 1 MHz. It is shown that some of the thermospheric and ionospheric parameters do not rapidly recover after the eclipse. In particular, T_n and the concentration of N₂ remained low above Chilton station until the end of the day. The diurnal variation in foF2 increases at 1800 UT compared to undisturbed conditions.

The unpredictable variability of the ionospheric F region greatly limits the efficiency of communications, radar and navigation systems which employ high frequency (HF) radiowaves. The objective of this work is to forecast the ionospheric critical frequency values (foF2) one hour in advance. For this a novel method has been developed for 1-hour ahead forecasting of the critical frequency of the F2 layer (foF2) based on applying feedback on predicted monthly median values of foF2 for each hour. The basic model for the prediction of the monthly medians consists of a parabolic dependency on R12 superimposed by a trigonometric expansion in terms of the harmonics of yearly variation, linearly modulated by R12. The monthly medians for each hour are predicted by applying the basic model over a sliding data window.

Intense late-cycle solar activity during October and November 2003 produced two strong geomagnetic storms: 28 October-5 November 2003 (October) and 1923 November 2003 (November); both reached intense geomagnetic activity levels, K_p = 9, and K_p = 8⁺, respectively. The October 2003 geomagnetic storm was stronger, but the effects on the Earth's ionosphere in the mid-latitude European sector were more important during the November 2003 storm. The aim of this paper is to discuss two significant effects observed on the ionosphere over the mid-latitude European sector produced by the November 2003 geomagnetic storm, using, data from ground ionosonde at Chilton (51.5 degrees N; 359.4 degrees E), Pruhonice (50.0 degrees N; 14.6 degrees E) and El Arenosillo (37.1 degrees N; 353.3 degrees E), jointly with GPS data. These effects are the presence of well developed anomalous storm E, layers observed at latitudes as low as 37 degrees N and the presence of two thin belts: one having enhanced electron content and other, depressed electron content. Both reside over the mid-latitude European evening sector.

The temporal and spatial behaviour of the ionospheric parameters foF2 and h'F during isolated substorms are examined using data from ionospheric stations distributed across Europe and western Asia. The main purpose is finding the forerunners of the substorm disturbances and a possible prediction of these disturbances. During the period from March 1998 to March 1999, 41 isolated substorms with intensities I = 60 - 400 nT were identified and studied. The study separated occasions when the local magnetometers were affected by the eastward electrojet (positive substorms) from those influenced by the westward electrojet (negative substorms). The deviations of the ionospheric parameters from their monthly medians (DfoF2 and Dh'F) have been used to determine the variations through the substorm. Substorm effects occurred simultaneously (< 1 h) across the entire observatory network. For negative substorms, DfoF2-values increase > 6 h before substorm onset, To, reaching a maximum 2-3 h before To. A second maximum occurs 1-2 h after the end of the substorm. The Dh'F values 3-4 h before To have a small minimum but then increase to a maximum at To. There is a second maximum at the end of the expansion phase before dh'F drops to a minimum 2-3 h after ending the expansion phase. For positive substorms, the timing of the first maximum of the dfoF2 and dh'F values depends on the substorm length - if it is longer, the position is closer to To. The effects on the ionosphere are significant: DfoF2 and Dh'F reach 2-3 MHz (dfoF2 = 50-70% from median value) and 50-70 km (D h'F = 20-30% from median value), respectively. Regular patterns of occurrence ahead of the first substorm signature on the magnetometer offer an excellent possibility to improve short-term forecasting of radio wave propagation conditions.

A knowledge of ionospheric propagation modes and signal strengths is important for the successful operation of HF point-to-point communication circuits and over-the-horizon radars. Predictions use representations of the state of the ionosphere based either on long term trends in past ionospheric data, or on near real time ionospheric soundings at vertical incidence or over oblique paths. A new prediction scheme is described which can be used with either forecast values or direct measurements of the standard ionospheric characteristics derived from vertical incidence soundings. Its important features include an improved model of the vertical distribution of electron concentration, a homing procedure to determine the rays which travel between specified terminals, an allowance for the focusing of rays with low elevation angles, an expression for ionospheric absorption based on the ionospheric characteristic foE and the inclusion of the effects of polarization coupling loss determined in terms of ray path and magnetic field geometry.

Instantaneous mapping techniques applied to geographically irregularly spaced foF2 measurements can lead sometimes to non-physical gradients. A procedure is presented to avoid such problems by the use of screen points within the area of interest having values derived from single station models (SSM's). Spatial smoothing uses the kriging method in terms of the deviations between the measurements and corresponding figures given by the adopted long-term mapping method of COST 238 (PRIME). A new first-order trough model is introduced as a correction to the mapped values on the equatorial side of the auroral oval by night. Sample maps of the European ionosphere generated by this technique are compared with internationally recommended monthly median prediction maps to demonstrate the lack of spatial structure these latter give, with consequential errors when applied to propagation assessments. The use of the new maps, particularly for the higher latitudes, is advocated.

Continuous observations in the ionospheric E and F regions have been regularly carried out since the fifties of this century at many ionosonde stations. Using these data from 31 European stations long-term trends have been derived for different parameters of the ionospheric E layer (h' E, foE), F1 layer (foF1) and F2 layer (hmF2, foF2). The detected trends in the E and F1 layers (lowering of the E region height h'E; increase of the peak electron densities of the E and F1 layers, foE and foF1) are in qualitative agreement with model predictions of an increasing atmospheric greenhouse effect. In the F2 region, however, the results are more complex. Whereas in the European region west of 30^oE negative trends in hmF2 (peak height of the F2 layer) and in the peak electron density (foF2) have been found, in the eastern part of Europe (east of 30^oE) positive trends dominate in both parameters. These marked longitudinal differences cannot be explained by an increasing greenhouse effect only, here probably dynamical effects in the F2 layer seem to play an essential role.

Basing on model calculations by Roble and Dickinson (1989) for an increasing content of atmospheric greenhouse gases in the Earth's atmosphere Rishbeth (1990) predicted a lowering of the ionospheric F2- and E-regions. Later Rishbeth and Roble (1992) also predicted characteristic longterm changes of the maximum electron density values of the ionospheric E-, F1-, and F2-layers. Long-term observations at more than 100 ionosonde stations have been analyzed to test these model predictions. In the E- and F1-layers the derived experimental results agree reasonably with the model trends (lowering of h0E and increase of foE and foF1, in the E-layer the experimental values are however markedly stronger than the model data). In the ionospheric F2-region the variability of the trends derived at the different individual stations for hmF2 as well as foF2 values is too large to estimate reasonable global mean trends. The reason of the large differences between the individual trends is not quite clear. Strong dynamical effects may play an important role in the F2-region. But also inhomogeneous data series due to technical changes as well as changes in the evaluation algorithms used during the long observation periods may influence the trend analyses.

The first part of the paper is directed to the investigation of the practical importance of possible longterm trends in the F2-layer for ionospheric prediction models. Using observations of about 50 different ionosonde stations with more than 30 years data series of foF2 and hmF2, trends have been derived with the solar sunspot number R-12 as index of the solar activity. The final result of this trend analysis is that the differences between the trends derived from the data of the individual stations are relatively large, the calculated global mean values of the foF2 and hmF2 trends, however, are relatively small. Therefore, these small global trends can be neglected for practical purposes and must not be considered in ionospheric prediction models. This conclusion is in agreement with the results of other investigations analyzing data of globally distributed stations. As shown with the data of the ionosonde station Tromso, however, at individual stations the ionospheric trends may be markedly stronger and lead to essential effects in ionospheric radio propagation. The second part of the paper deals with the reasons for possible trends in the Earth's atmo- and ionosphere as investigated by different methods using characteristic parameters of the ionospheric D-, E-, and F-regions. Mainly in the F2-region different analyses have been carried out. The derived trends are mainly discussed in connection with an increasing greenhouse effect or by long-term changes in geomagnetic activity. In the F I-layer the derived mean global trend in foF1 is in good agreement with model predictions of an increasing greenhouse effect. In the E-region the derived trends in foE and h'E are compared with model results of an atmospheric greenhouse effect, or explained by geomagnetic effects or other anthropogenic disturbances. The trend results in the D-region derived from ionospheric reflection height and absorption measurements in the LF, MF and HF ranges can at least partly be explained by an increasing atmospheric greenhouse effect.

New ionospheric activity indices are derived from automatically scaled online data from several European ionosonde stations. These indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). The most reliable indices are derived from the maximum electron density of the ionospheric F2-layer expressed by the maximum critical frequency foF2. Similar indices derived from ionospheric M(3000)F2 values show a markedly lower variability indicating that the changes of the altitude of the F2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the F 2-layer. By using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of Europe (34^oN 60^oN; 5^oW 40^oE) can now be displayed online.

The structure of the Interplanetary Magnetic Field (IMF) is responsible for an essential part of the variability of the ionospheric plasma as demonstrated by investigations of the influence of IMF sector boundary crossings as well as of B_z-changes (defined from satellite observations) to the maximal electron density of the F2-layer at different stations in mid-latitudes. It could be shown that negative B_z-values cause distinct negative ionospheric effects. Maximal effects were detected at high geomagnetic latitudes (ionospheric response decreases with decreasing latitude), high solar/geomagnetic activity, equinoxes and night-time conditions.

Detailed studies of the development of photospheric activity centres for two solar cycles show that Spoerer's Law holds in a very similar form to that applying to sunspots for the faculae which inhabit the sunspot zones. Similar differences between the two solar hemispheres can arise, and it seems to be confirmed that the average latitude of faculae tends to be a few degrees poleward of that of sunspots throughout a given cycle. It is shown that the normal averaging process involved in deriving Spoerer's Law obscures a detail which is revealed in a breakdown into the variations within successive narrow latitude strips. These show the existence within a cycle of three separate maxima of activity occurring at different epochs and with different preferred latitudes. The main properties of these maxima are discussed.

The hysteresis of foF2 is studied for several European stations over the whole 24-hour diurnal interval for the equinoctial months of the years just before and just after the solar cycle minimum for solar cycles 20 and 21. Based on previous results, the hysteresis is expected to develop best just for the equinoctial months and near the solar cycle minimum. The hysteresis is generally found to be negative, i.e. higher foF2 for the rising branch compared to the falling branch of solar cycle. However, this is not the case in some individual months of some years. The noontime hysteresis represents the hysteresis at other times of the day qualitatively (as to sign) but not quantitatively. The hysteresis appears to be relatively persistent from one solar cycle to another solar cycle in spring but not in autumn. A typical value for springtime hysteresis is about 0.5 MHz. The inclusion of hysteresis into longterm ionospheric and radio wave propagation predictions remains questionable.

This paper attempts to demonstrate the changes in the F1 layer ionization during geomagnetic storm. To analyze the behavior of F1 region, we have selected eight rather strong geomagnetic storms that occurred in different seasons in 1994-1997. Their course was similar and there were at least three quiet days before each event. The electron density profiles for these events, derived from all the available ionograms of the Pruhonice station (50 degrees N, 14.6 degrees E), were analyzed in order to investigate electron density variability at heights of 160-190 km. Spring/autumn asymmetry of the effects in F1 region is found. We observed no significant effect of an ionospheric storm in electron density in the F1 region during spring geomagnetic storms, while there is a substantial effect in autumn at 180 and 190 km heights. We have compared our results with those obtained from ionograms of some other European ionospheric stations. In general, the F1 region appears to be much more stable than the F2 layer during ionospheric storms. Substantial intra-hour variability was found in NmF2 during geomagnetic storms in daytime, while it was very weak on the storm maximum day in F1 layer.

This study attempts to demonstrate changes in the ionospheric F1-region daytime ionization during geomagnetic storms. The F1-region is explored using available data from several European middle latitude and lower latitude observatories and a set of geomagnetic storms encompassing a range of seasons and solar activity levels. The results of analysis suggest systematic seasonal and partly latitudinal differences in the F1-region response to geomagnetic storm. The pattern of the response of the F1-region at higher middle latitudes, a decrease in electron density, does not depend on the type of response of the F2-region and on solar activity. A brief interpretation of these findings is presented.

With the advent of the Global Positioning System (GPS) measurements (from both ground-based and satellite-based receivers), the number of available ionospheric measurements has dramatically increased. Total electron content (TEC) measurements from GPS instruments augment observations from more traditional ionospheric instruments like ionospheric sounders and Langmuir probes. This volume of data creates both an opportunity and a need for the observations to be collected into coherent synoptic scale maps. This paper describes the Ionospheric Data Assimilation Three-Dimensional (IDA3D), an ionospheric objective analysis algorithm. IDA3D uses a three-dimensional variational data assimilation technique (3DVAR), similar to those used in meteorology. IDA3D incorporates available data, the associated data error covariances, a reasonable background specification, and the expected background error covariance into a coherent specification on a global grid. It is capable of incorporating most electron density related measurements including GPS-TEC measurements, low-Earth-orbiting "beacon" TEC, and electron density measurements from radars and satellites. At present, the background specification is based upon empirical ionospheric models, but IDA3D is capable of using any global ionospheric specification as a background. In its basic form, IDA3D produces a spatial analysis of the electron density distribution at a specified time. A time series of these specifications can be created using past specifications to determine the background for the current analysis. IDA3D specifications are able to reproduce dynamic features of electron density, including the movement of the auroral boundary and the strength of the trough region.

The daily variability of the ionosphere can greatly affect HF or SATCOM communications. HF skywave operators plan frequency schedules months in advance, however, they also require daily knowledge of the ionospheric conditions in order to modify assignments. SATCOM operators also require daily information about the levels of scintillation, which are variations in phase, amplitude, polarisation and angle of arrival that can cause severe degradation of the received signal. Using a number of ionosonde measurements and geomagnetic and solar values, a Daily Ionospheric Forecasting Service (DIFS) has been developed, which provides HF and SATCOM operators with daily forecasts of predicted ionospheric conditions. The system uses in-house algorithms and an externally developed Global Ionospheric Scintillation Model (GISM) to generate HF and SATCOM forecasts. HF forecasts consist of a past summary and a forecast section, primarily displaying observed values and predicted categories for the Maximum Usable Frequency (MUF), as well as an Ionospheric Correction factor (ICF) that can be fed into the ionospheric propagation prediction tool, WinHF. SATCOM forecasts give predictions of global scintillation levels, for the polar, mid and equatorial latitude regions. Thorough analysis was carried out on DIFS and the results conclude that the service gives good accuracy, with user feedback also confirming this, as well.

Past COST projects 238 (PRIME) and 251 (IITS) encompassed a variety of efforts to forecast and map the ionospheric plasma response to disturbed geophysical conditions over the European region. In a number of case studies they provided useful guidelines for what could be achieved by different theoretical, empirical and artificial neural network techniques. New COST 271 action on 'Effects of the upper atmosphere on terrestrial and Earth-space communications' which began in the early months of the year 2000 is focusing upon efforts to map the ionospheric space weather conditions over Europe in near-real-time and to forecast these conditions a few hours ahead. The purpose of the paper is to present some of the findings from the previous COST 251 study and the plans for the new one (COST 271).

Real-time measurements of the critical frequency of F-2 layer, foF(2), and the propagation factor for a 3000 kin range, M(3000)F2 from four European Digisondes operating in Athens, Rome, Chilton and Juliusruh and the Bz-component of the interplanetary magnetic field, Bz-IMF, from the NASA Advanced Composition Explorer (ACE) spacecraft mission are combined for the development of a real-time dynamic system, oriented to monitor the ionospheric propagation conditions over Europe. The validity of the developed system in its present operational form is investigated through the analysis of two case study events. First results indicate a temporal correlation between the Bz-IMF component disturbances and the quantitative signature of ionospheric disturbances at middle latitude, making the developed facility a useful tool for modelling. and forecasting ionospheric propagation conditions.

Characteristics of midlatitude ionospheric disturbances during several great geomagnetic storms have been investigated using data from the European geomagnetic observatories and ionospheric stations with the aim of developing the local forecasting models, as part of the prediction and retrospective ionospheric modeling over Europe project. Based on the analysis of the geomagnetic storms of February 6, 1986, and March 13, 1989, a detailed picture of the local H component of geomagnetic field and the ionospheric critical frequency f0F2 variations is presented. The results show that f0F2 was dramatically changed above or below the monthly median level in a relatively narrow band about 15^o of latitude and 30^o longitude during the different phases of the storms. These results support the view that day-to-day F region ionospheric variability is essentially altered in great storms. Consequences of those effects for short-term modeling purposes are discussed.

A prominent large-scale ionospheric disturbance was observed in the European mid-latitude sector during the recent extreme space weather event in October 2003. Measurements of the horizontal component of the geomagnetic field H, the critical frequency of the F2 layer foF2, and the vertical total electron content (TEC) from the European network of observational sites are used to describe the temporal and spatial storm evolution process. It is found that the ionospheric F region storm morphology was dominated by negative disturbances over high mid-latitudes and positive disturbance at low mid-latitudes during the initial phase and by overall negative disturbances during the main phase. Although a good agreement between the two independent measurements was detected by comparing the storm-time behaviour of foF2 and TEC during the main phase of the storm, some irregularities have been recognised in TEC variations at high mid-latitudes. The relative merit of real-time observational solar-terrestrial data for accurate specification of the geomagnetically disturbed ionospheric F region during the extreme space weather conditions is discussed.

An artificial neural network method is applied to the development of an ionospheric forecasting technique for one hour ahead. Comparisons between the observed and predicted values of the critical frequency of the F 2 layer, foF2, and the total electron content (TEC) are presented to show the appropriateness of the proposed technique.

A nonlinear technique employing radial basis function neural networks (RBF-NNs) has been applied to the short-term forecasting of the ionospheric F2-layer critical frequency, foF2. The accuracy of the model forecasts at a northern mid-latitude location over long periods is assessed, and is found to degrade with time. The results highlight the need for the retraining and re-optimization of neural network models on a regular basis to cope with changes in the statistical properties of geophysical data sets. Periodic retraining and re-optimization of the models resulted in a reduction of the model predictive error by similar to 0.1 MHz per six months. A detailed examination of error metrics is also presented to illustrate the difficulties encountered in evaluating the performance of various prediction/forecasting techniques.

The longest data sets available for estimating thermospheric temperature trends are those from ground-based ionosondes, which often begin during the International Geophysical Year of 1957, close to a solar activity maximum. It is important to investigate inconsistencies in trend estimates from these data sets so that trends can be clearly determined. Here we use selected ionosonde stations to show that one of the most significant factors affecting the trend estimates is the removal of the solar cycle. The stations show trend behavior that is close to the behavior of a theoretical model of damped harmonic oscillation. The ringing features are consistent with the presence of solar cycle residuals from the analysis with an amplitude of 2.5 km. Some stations do not show trend behavior that is close to either the average behavior of the stations studied here or the theoretical model of oscillation. Four European stations (Poitiers, Lannion, Juliusruh, and Slough), three of which are closely located in western Europe, were analyzed with the expectation that their trend should be similar. Only Poitiers and Juliusruh showed an evolution that was close to the average behavior of other stations, while the other two were significantly different. The primary cause of this appears to be changes in the M(3000)F2 parameter and demonstrates the importance of incorporating consistency checks between neighboring ionosondes into global thermospheric trend estimates.

The robustness of the aa geomagnetic index is of critical importance to the debate about the previously reported doubling of the solar coronal magnetic field in the last 100 years, inferred from an increasing trend in this index. To test the trend in aa, we have reconstructed the aa index using two long-running European stations (Sodankyla from 1914 and Niemegk from 1890) to provide data for the northern component of the index that are independent of data from the UK observatories used in the "official” aa index. Both the fully "reconstructed” aa series, based on Sodankyla ( 67 degrees N, L = 5.2 R_E) and Niemegk ( 52 degrees N, L = 2.3 R_E) data in combination with the official aa Southern Hemisphere data, confirm the increasing trend in the index. The Niemegk-based index shows little solar cycle variation in its deviation from the official index, probably because of the midlatitude location of the station. The high-latitude station, Sodankyla, is more affected by active geomagnetic conditions during solar maximum because of the proximity of the auroral oval to the station. Nevertheless, its index also clearly confirms the increasing trend in the aa index and hence supports the idea of a long-term increase in solar coronal magnetic field strength. As an added test, we reconstructed the aa index from a single site using data from two long-running UK stations, Eskdalemuir and Lerwick, applying a technique known as interhourly variation (IHV) proposed by Svalgaard et al. (2004). The resulting series is designed to be primarily sensitive to solar wind conditions. Both the reconstructed aa(IHV) also showed an increasing trend with time and high consistency with the official aa index. Overall, we conclude that the robustness of the trend in the aa index supports the idea of a long-term increase in solar coronal magnetic field strength.

The asymmetries in the convective flows, current systems, and particle precipitation in the high-latitude dayside ionosphere which are related to the equatorial plane components of the interplanetary magnetic field (IMF) are discussed in relation to the results of several recent observational studies. It is argued that all of the effects reported to date which are ascribed to the y component of the IMF can be understood, at least qualitatively, in terms of a simple theoretical picture in which the effects result from the stresses exerted on the magnetosphere consequent on the interconnection of terrestrial and interplanetary fields. In particular, relaxation under the action of these stresses allows, in effect, a partial penetration of the IMF into the magnetospheric cavity, such that the sense of the expected asymmetry effects on closed field lines can be understood, to zeroth order, in terms of the "dipole plus uniform field" model. In particular, in response to IMF By, the dayside cusp should be displaced in longitude about noon in the same sense as By in the northern hemisphere, and in the opposite sense to By in the southern hemisphere, while simultaneously the auroral oval as a whole should be shifted in the dawn-dusk direction in the opposite sense with respect to By. These expected displacements are found to be consistent with recently published observations. Similar considerations lead to the suggestion that the auroral oval may also undergo displacements in the noon-midnight direction which are associated with the x component of the IMF. We show that a previously published study of the position of the auroral oval contains strong initial evidence for the existence of this effect. However, recent results on variations in the latitude of the cusp are more ambiguous. This topic therefore requires further study before definitive conclusions can be drawn.

It is important to correctly quantify the effect of solar forcing of climate to gain a better understanding of the factors that influence the overall climate system. Using an "optimal fingerprinting" technique we find that current climate models underestimate the observed climate response to solar forcing over 11-year timescales, indicating that the real climate system has a greater sensitivity to solar forcing than current climate models responding to changes in external TSI alone. Consideration of two sets of HadCM2 ensembles, each forced with a different reconstruction of solar irradiance was employed in this analysis. In both scenarios the simulated response is much weaker than the observed, implying a need for a deeper understanding of solar-climate interactions. This discrepancy is could be attributable to a variation in the Earth's albedo over periods in phase with characteristic solar timescales. The proposed "Earthshine" mission seeks to investigate this possibility using an instrument located at the L1 Lagrangian point. Results from a preliminary signal-to-noise analysis with a consideration of differing sections of the 11-year solar cycle will be presented to show that a solar induced variation in albedo could be detected by such an instrument.

Models of the ionosphere, used in applications for the prediction or correction of propagation effects on practical radio systems, are often inadequate in their representation of the structure and development of large-scale features in the electron density. Over northern Europe, characterization of the main trough presents particular problems for such empirical or parameterized models and hence for radio propagation forecasting and ionospheric mapping. Results are presented from a study aimed at investigating the possible role of radio tomographic imaging in adapting models to yield a better representation of the ionosphere over Europe. It is shown that use of radio tomography gives better agreement with actual ionosonde data than can be obtained from any of the models used alone. It is suggested that the technique may have a possible role in the mapping of ionospheric conditions in near-real time for future systems applications.

The F2-region response to a geomagnetic storm usually called a ionospheric storm is a rather complicated event. It consists of the so-called positive an negative phases, which have very complicated spatial and temporal behavior. During the recent decade there was significant progress in understanding this behavior. The principal features of the positive and negative phase distribution and variations have been explained on the basis of the principal concept: during a geomagnetic disturbance there is an input of energy into the polar ionosphere, which changes thermospheric parameters, such as composition, temperature and circulation. Composition changes directly influence the electron concentration in the F2 region. The circulation spreads the heated gas to lower latitudes. The conflict between the storm-induced circulation and the regular one determines the spatial distribution of the negative and positive phases in various seasons. There are still problems unsolved. The most acute ones are: the appearance of positive phases before the beginning of a geomagnetic disturbance, the occurrence of strong negative phases at the equator, the role of vibrationally excited nitrogen in forming the negative phase, and the relation of positive phases to the dayside cusp. There are indications that the f(o)F2 long-term trends revealed during the recent years may be explained by long-term trends of the number of negative ionospheric disturbances due to secular variations of the geomagnetic activity.

A detailed analysis of the foF2 data at a series of ionospheric stations is performed to reveal long-term trends independent of the long-term changes in geomagnetic activity during the recent decades (nongeomagnetic trends). The method developed by the author and published earlier is used. It is found that the results for 21 out of 23 stations considered agree well and give a relative nongeomagnetic trend of -0.0012 per year (or an absolute nongeomagnetic trend of about -0.012 MHz per year) for the period between 1958 and the mid-nineties. The trends derived show no dependence on geomagnetic latitude or local time, a fact confirming their independence of geomagnetic activity. The consideration of the earlier period (1948-1985) for a few stations for which the corresponding data are available provides significantly lower foF2 trends, the difference between the later and earlier periods being a factor of 1.6. This is a strong argument in favor of an anthropogenic nature of the trends derived.

The ionospheric sounding data at two southern hemisphere stations, the Argentine Islands and Port Stanley, are analyzed using a method previously developed by the authors. Negative trends of the critical frequency foF2 are found for both stations. The magnitudes of the trends are close to those at the corresponding (dose geomagnetic latitude) stations of the northern hemisphere, as considered previously by the authors. The values of the F2 layer height hmF2 absolute trends Delta hmF2 are considered. The effect of Delta hmF2 dependence on hmF2 found by Jarvis et al. (1998) is reproduced. A concept is considered that long-term changes of the geomagnetic activity may be an important (if not the only) cause of all the trends of foF2 and hmF2 derived by several groups of authors. The dependence of both parameters on the geomagnetic index Ap corresponds to a smooth scheme of the ionospheric storm physics and morphology; thus, a principal cause of the foF2 and hmF2 geomagnetic trends is most probably a trend found in several publications in the number and intensity of ionospheric storms.

The data of vertical ionospheric sounding at Argentine Island and Port Stanley stations in the Southern Hemisphere are analyzed using the method of long-term trends developed by us earlier. The negative trends in the critical frequency f(0)F2 have been found for both stations. The trend magnitudes are similar to such magnitudes at stations located at close geomagnetic latitudes in the Northern Hemisphere and considered by us earlier. The values of the absolute trends in the F2 layer height (hmF2, Delta hmF2) are considered. The effect of the Delta hmF2 dependence on hmF2 determined by Jarvis et al. [1998] is reproduced. A conclusion is drawn that all trends in f(0)F2 and hmF2 derived by different groups of authors have a geomagnetic origin and are a manifestation of the long-term changes in the geomagnetic activity. It has been shown that the dependence of both parameters on the geomagnetic index Ap corresponds to a smoothed scheme of the physics and morphology of the ionospheric storms. The trends in both ionospheric parameters (f(0)F2 and hmF2) apparently reflect the long-term trends in the number and intensity of the ionospheric storms found in several publications.

Superposed epoch studies have been carried out in order to determine the ionospheric response at mid-latitudes to southward turnings of the interplanetary magnetic field (IMF). This is compared with the geomagnetic response, as seen in the indices K_p, AE and Dst. The solar wind, IMF and geomagnetic data used were hourly averages from the years 1967–1989 and thus cover a full 22-year cycle in the solar magnetic field. These data were divided into subsets, determined by the magnitudes of the southward turnings and the concomitant increase in solar wind pressure. The superposed epoch studies were carried out using the time of the southward turning as time zero. The response of the mid-latitude ionosphere is studied by looking at the F-layer critical frequencies, f_oF2, from hourly soundings by the Slough ionosonde and their deviation from the monthly median values, δf_oF2. For the southward turnings with a change in B_z of δB_z>11.5 nT accompanied by a solar wind dynamic pressure P exceeding 5 nPa, the F region critical frequency, f_oF2, shows a marked decrease, reaching a minimum value about 20 h after the southward turning. This recovers to pre-event values over the subsequent 24 h, on average. The Dst index shows the classic storm-time decrease to about -60 nT. Four days later, the index has still to fully recover and is at about -25 nT. Both the K_p and AE indices show rises before the southward turnings, when the IMF is strongly northward but the solar wind dynamic pressure is enhanced. The average AE index does register a clear isolated pulse (averaging 650 nT for 2 h, compared with a background peak level of near 450 nT at these times) showing enhanced energy deposition at high latitudes in substorms but, like K_p, remains somewhat enhanced for several days, even after the average IMF has returned to zero after 1 day. This AE background decays away over several days as the Dst index recovers, indicating that there is some contamination of the currents observed at the AE stations by the continuing enhanced equatorial ring current. For data averaged over all seasons, the critical frequencies are depressed at Slough by 1.3 MHz, which is close to the lower decile of the overall distribution of δf_oF2 values. Taking 30-day periods around summer and winter solstice, the largest depression is 1.6 and 1.2 MHz, respectively. This seasonal dependence is confirmed by a similar study for a Southern Hemisphere station, Argentine Island, giving peak depressions of 1.8 MHz and 0.5 MHz for summer and winter. For the subset of turnings where δB_z>11.5 nT and P<= 5 nPa, the response of the geomagnetic indices is similar but smaller, while the change in δf_oF2 has all but disappeared. This confirms that the energy deposited at high latitudes, which leads to the geomagnetic and ionospheric disturbances following a southward turning of the IMF, increases with the energy density (dynamic pressure) of the solar wind flow. The magnitude of all responses are shown to depend on δB_z. At Slough, the peak depression always occurs when Slough rotates into the noon sector. The largest ionospheric response is for southward turnings seen between 15–21 UT.

Measurements of the ionospheric E region during total solar eclipses in the period 1932-1999 have been used to investigate the fraction of Extreme Ultra Violet and soft X-ray radiation, phi, that is emitted from the limb corona and chromosphere. The relative apparent sizes of the Moon and the Sun are different for each eclipse, and techniques are presented which correct the measurements and, therefore, allow direct comparisons between different eclipses. The results show that the fraction of ionising radiation emitted by the limb corona has a clear solar cycle variation and that the underlying trend shows this fraction has been increasing since 1932. Data from the SOHO spacecraft are used to study the effects of short-term variability and it is shown that the observed long-term rise in phi has a negligible probability of being a chance occurrence.

A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s(1). Several mechanisms have been proposed to explain this connection(2-7), and evidence from modelling(8) as well as various types of measurements(9-14) demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events(15), that the ionospheric 'sporadic E' layer - transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation - can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.

Swept-frequency (1-10 MHz) ionosonde measurements were made at Helston, Cornwall (50^o06'N, 5^o18'W) during the total solar eclipse on August 11, 1999. Soundings were made every three minutes. We present a method for estimating the percentage of the ionising solar radiation which remains unobscured at any time during the eclipse by comparing the variation of the ionospheric E-layer with the behaviour of the layer during a control day. Application to the ionosonde date for 11 August, 1999, shows that the flux of solar ionising radiation fell to a minimum of 25±2% of the value before and after the eclipse. For comparison, the same technique was also applied to measurements made during the total solar eclipse of 9 July, 1945, at Sörmjöle (63^o68'N, 20^o20'E) and yielded a corresponding minimum of 16±2%. Therefore the method can detect variations in the fraction of solar emissions that originate from the unobscured corona and chromosphere. We discuss the differences between these two eclipses in terms of the nature of the eclipse, short-term fluctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic field.

Long timeseries of the critical frequency of the F2 layer, foF2, from several mid- and high-latitude stations, are used for investigating the average behaviour of the disturbed ionospheric conditions identified by descriptive letters replacing or accompanying the ionogram scaled value. By analysing the distribution of the descriptive letters A (sporadic Es layer presence), B (absorption near fmin ), spread F appearance, G (screen by F1 layer) and R (absorption near foF2), the mean percentage occurrence of the ionospheric irregularities are calculated for four specified levels of magnetic activity according to a new magnetic activity catalogue (MAC) recently introduced for studying the dependence of the ionosphere on magnetic perturbations. After removing solar cycle effects from the statistical results obtained, it is found that the total bottomside irregularities increase with geomagnetic latitudes and represent an indicator of the ionospheric response to the magnetic activity with a time delay of the order of about 15 h according to the MAC.

The noon median values of the E-layer critical frequency (foE) measured at Slough/Chilton (1931-1997), Moscow (1946-1997), and Tomsk (1938-1997) stations have been analyzed. New regularities in the foE climatic (long-term) variations, the regression dependences of these variations on the Wolf numbers averaged over 11 years (R-11, a global factor), and the surface air temperature near a particular station minus the temperature at the ocean-continent boundary (DeltaT(11), a regional factor) have been determined. The global factor predominates for Slough/Chilton station located in the vicinity of the ocean-continent boundary. The additional regression dependence of foE on DeltaT(11) is substantial and significant for the continental stations (the continental effect). For Tomsk, this effect is even a predominant cause of climatic variations in foE.

The effects of changing solar EUV flux upon the location and structure of the dayside high-latitude trough are investigated, using a combination of modelling studies and experimental ionospheric tomography. An increase in the EUV radiation incident upon the atmosphere causes thermal expansion, and an increase in ionisation. This drives ionospheric features such as troughs to a greater height. However, such behaviour is further complicated by increased solar activity changing the chemical composition of the atmosphere, and the fact that many of the chemical reactions of importance to the ionosphere are temperature dependent. Since a complete understanding of the variation of ionospheric temperature with solar activity remains elusive, the effects of the above processes upon the dayside high-latitude trough cannot be predicted. In the present paper results from theoretical modelling using the Coupled Thermosphere-Ionosphere-Plasmasphere model are compared with tomographic observations of the ionosphere. Calculations have been performed for low, medium and high solar activity and compared with tomographic reconstructions of ionospheric electron density, taken under broadly similar conditions, when geomagnetic activity was low. The results show that the trough is driven polewards in latitude, and that the density gradient equatorwards of the trough increases significantly, as solar activity increases. However, the basic structure and form of the trough are little changed. There is broad agreement between the CTIP modelling calculations and the experimental tomography observations.

High-speed solar wind streams (HSSs) are periods of persistently high solar wind, which emanate from coronal holes and may recur with a frequency related to the solar rotation period of 27 days. On arrival at the Earth's magnetopause, such streams cause a series of events which ultimately lead to changes in the ionospheric F layer. We present a superposed epoch analysis of parameters in the midlatitude F2 layer for a collection of 124 high-speed solar wind streams which occurred between 1993 and 2006. Clear changes in the critical frequency (foF2), density (NmF2), and height (hmF2) are found to occur after the onset of magnetospheric convection associated with HSS arrival at the Earth's magnetosphere. A fall in foF2 occurs immediately following convection onset accompanied by a sudden decrease in NmF2 and an increase in hmF2. During the events under study, the height of the F2 layer is found to increase by ∼20 km at convection onset. A period of more than 4 days is required for the ionosphere to return to preevent levels. This behavior is explained as the occurrence of ionospheric F region storms following HSS arrival. The results raise the possibility of improved predictions for ionospheric parameters on the basis of upstream solar wind conditions and prior identification of stream interfaces.

A new sunspot and faculae digital dataset for the interval 1874–1955 has been prepared under the auspices of the NOAA National Geophysical Data Center (NGDC). This digital dataset contains measurements of the positions and areas of both sunspots and faculae published initially by the Royal Observatory, Greenwich, and subsequently by the Royal Greenwich Observatory (RGO), under the title Greenwich Photo-heliographic Results (GPR), 1874–1976. Quality control (QC) procedures based on logical consistency have been used to identify the more obvious errors in the RGO publications. Typical examples of identifiable errors are North versus South errors in specifying heliographic latitude, errors in specifying heliographic (Carrington) longitude, errors in the dates and times, errors in sunspot group numbers, arithmetic errors in the summation process, and the occasional omission of solar ephemerides. Although the number of errors in the RGO publications is remarkably small, an initial table of necessary corrections is provided for the interval 1874–1917. Moreover, as noted in the preceding companion papers, the existence of two independently prepared digital datasets, which both contain information on sunspot positions and areas, makes it possible to outline a preliminary strategy for the development of an even more accurate digital dataset. Further work is in progress to generate an extremely reliable sunspot digital dataset, based on the long programme of solar observations supported first by the Royal Observatory, Greenwich, and then by the Royal Greenwich Observatory.

The 11 August 1999 Solar eclipse totality path ran across western Europe at near-constant latitudes of about 49 degreesN. It occurred at mid-time of a sequence of three days with steady solar wind and quiet magnetospheric conditions. Its response was observed by a score of ionospheric facilities, which will provide high-resolution probing of the various disturbances. First results allow us to compare the time fluctuations at various distances from totality on the eclipse and adjacent days, inside a 5 degrees West to 5 degrees East longitude area. In this preliminary work the foF1 and foF2 time changes are presented in contour maps on a 50 km size grid. They show the expected longitude transit of eclipse perturbation. We venture brief comments on the eclipse-own signatures as separate from the various wave oscillations detected prior to eclipse time by 12.4 MHz panoramic azimuth scans of the Losquet radar near Lannion (Brittanny).

The terrestrial magnetopause suffered considerable sudden changes in its location on 9-10 September 1978. These magnetopause motions were accompanied by disturbances of the geomagnetic field on the ground. We present a study of the magnetopause motions and the ground magnetic signatures using, for the latter, 10 s averaged data from 14 high latitude ground magnetometer stations. Observations in the solar wind (from IMP 8) are employed and the motions of the magnetopause are monitored directly by the spacecraft ISEE 1 and 2. With these coordinated observations we are able to show that it is the sudden changes in the solar wind dynamic pressure that are responsible for the disturbances seen on the ground. At some ground stations we see evidence of a “ringing” of the magnetospheric cavity, while at others only the initial impulse is evident. We note that at some stations field perturbations closely match the hypothesized ground signatures of flux transfer events. In accordance with more recent work in the area (e.g. Potemra et al., 1989, J. geophys. Res., in press), we argue that causes other than impulsive reeonnection may produce the twin ionospheric flow vortex originally proposed as a flux transfer even signature.

Recent research by on the am geomagnetic data series, recording of which began in 1958, suggests that the Russell-McPherron (RM) effect is not responsible for the majority of annual and diurnal variations detected in geophysical indices, rather a so-called "equinoctial effect" is dominant. We demonstrate a simple conversion of the aa index (which, with only two antipodal stations, has difficulty resolving diurnal variations) into a proxy am index, allowing us to extend the analysis back to 1868. The case for the equinoctial effect becomes less compelling as we examine the earlier data. Additionally examination of in situ solar wind measurements shows that the RM effect is clearly visible in geophysical indices for slow solar wind but not for fast. Sargent's recurrence index is used to extend these results to the extended am index.

There are no direct observational methods for determining the total rate at which energy is extracted from the solar wind by the magnetosphere. In the absence of such a direct measurement, alternative means of estimating the energy available to drive the magnetospheric system have been developed using different ionospheric and magnetospheric indices as proxies for energy consumption and dissipation and thus the input. The so-called coupling functions are constructed from the parameters of the interplanetary medium, as either theoretical or empirical estimates of energy transfer, and the effectiveness of these coupling functions has been evaluated in terms of their correlation with the chosen index. A number of coupling functions have been studied in the past with various criteria governing event selection and timescale. The present paper contains an exhaustive survey of the correlation between geomagnetic activity and the near-Earth solar wind and two of the planetary indices at a wide variety of timescales. Various combinations of interplanetary parameters are evaluated with careful allowance for the effects of data gaps in the interplanetary data. We show that the theoretical coupling, P_α, function first proposed by Vasyliunas et al. is superior at all timescales from 1-day to 1-year.

Hourly foF2 data from over 100 ionosonde stations during 1967-89 are examined to quantify F-region ionospheric variability, and to assess to what degree the observed variability may be attributed to various sources, i.e., solar ionizing Aux, meteorological influences, and changing solar wind conditions. Our findings are as follows. Under quiet geomagnetic conditions (K_p < 1), the 1-sigma (sigma is the standard deviation) variability of N-max about the mean is approx. ±25-35% at 'high frequencies' (periods of a few hours to 1-2 days) and approx. ±15-20% at 'low frequencies' (periods approx. 2-30 days), at all latitudes. These values provide a reasonable average estimate of ionospheric variability mainly due to "meteorological influences" at these frequencies. Changes in N-max due to variations in solar photon flux, are, on the average, small in comparison at these frequencies. Under quiet conditions for high-frequency oscillations, N-max is most variable at anomaly peak latitudes. This may reflect the sensitivity of anomaly peak densities to day-to-day variations in F-region winds and electric fields driven by the E-region wind dynamo. Ionospheric variability increases with magnetic activity at all latitudes and for both low and high frequency ranges, and the slopes of all curves increase with latitude. Thus, the responsiveness of the ionosphere to increased magnetic activity increases as one progresses from lower to higher latitudes. For the 25% most disturbed conditions (K_p > 4), the average 1-sigma variability of N-max about the mean ranges from approx. ±35% (equator) to approx. ±45% (anomaly peak) to approx. ±55% (high-latitudes) for high frequencies, and from approx. ±25% (equator) to approx. ±45% (high-latitudes) at low frequencies. Some estimates are also provided on N-max variability connected with annual, semiannual and Ii-year solar cycle variations.

Using sunspot observations from Greenwich and Mount Wilson, we show that the latitudinal spread of sunspot groups has increased since 1874, in a manner that closely mirrors the long-term ( 100 year) changes in the coronal source flux, F_S, as inferred from geomagnetic activity. This latitude spread is shown to be well correlated with the flux emergence rate required by the model of the coronal source flux variation by Solanki et al. [2000]. The time constant for the decay of this open flux is found to be 3.6±0.8 years. Using this value, and quantifying the photospheric flux emergence rate using the latitudinal spread of sunspot groups, the model reproduces the observed coronal source flux variation. The ratio of the 100-year drift to the solar cycle amplitude for the flux emergence rate is found to be half of the same ratio for F_S.

When modelling the evolution of emerged magnetic flux threading the solar surface, and the open solar flux that results, two factors are critical for each newly-emerged bipolar magnetic region (BMR), namely its latitude and the tilt angle of the line connecting the centres of the two opposite polarity regions. The variation of the former throughout the solar cycle is given by the well-known butterfly diagram, however the behaviour of the latter is not so clearly defined. Using magnetogram observations of BMRs, a systematic variation of average tilt angle with heliographic latitude, and thus with the solar cycle phase, has been reported and used in several modelling studies. However, using observations of sunspot pairs no such variation is apparent. We here investigate various subsets of the tilt angle data from sunspots in an attempt to reconcile and understand these apparently contradictory results.

Monthly values of coronal source flux have been created dating back until 1868 using the geomagnetic aa index (Lockwood et al, 1999). These values have been found to correlate well with the composite solar irradiance variation compiled from measurements by Virgo, Acrim I and II, HF and ERBS instrumentation (Frohlich and Lean, 1998). Using the monthly values of F_S as a proxy for solar irradiance, we were able to reconstruct solar irradiance back to 1868. We have also created a model of PSI (photometric sunspot index) dating back until 1874, using sunspot area as a proxy. Combing both of these models we have been able to reconstruct the facular bright- ening (including quiet sun) variations since 1874. We also continue our investigation using annual values of coronal source flux and PSI (from sunspot number) to look at facular behaviour at all times since the end of the Maunder minimum.

Using Greenwich sunspot records and a model of open flux evolution for individual bipolar magnetic regions (BMRs) we investigate reconstructing the variations of the total open solar flux and it's rate of emergence for the interval 1874 - 1981. Because it deals only with the evolution of individual BMRs the model includes only destruction of open flux at the BMR polarity reversal and not that at neutral lines between individual BMRs when more than one is present. The reconstruction makes use of the greenwich records of the appearance of sunspot groups and the model is then used along with the group latitude and umbral area to predict how the open flux would evolve if there was no inter-BMR loss. Using a super-posed epoch technique we compare the average evolution of sunspot area with that predicted by the model the differnce telling us about the effect of loss of total flux at inter-BMR neutral lines. From this we can correct the predicted open flux variation to allow for the effect of the inter-BMR neutral lines. The interval of the greenwich data is important because we can compare the results with the open flux variations calculated by Lockwood et al. [1999].

On the basis of an ionospheric definition of disturbed conditions independent of any causative mechanism, a feature-guided pattern recognition method reveals the dominant morphology of long-duration negative foF2 disturbances. A catalogue of negative disturbances lasting more than 24 hours is compiled from hourly foF2 data from 75 ionosonde stations and three solar cycles. Disturbances in each month and station are handled separately, and four local time intervals of disturbance commencement are considered. A median disturbance profile is produced only when a minimum occurrence probability holds. The time window under morphological investigation is selected such that nonsystematic features, precursor phenomena, and poststorm effects are not included in analysis. The disturbance patterns, first grouped according to major characteristic features and then fitted with simple mathematic functions, are described by a range of the normalized deviation of hourly foF2 to its corresponding monthly median and are provided to radio users along with their distribution in space and time. The present model is a nonconditional stand-alone model which may, in the event of an ionospheric disturbance at a certain location, predict its further development.

A technique for neural network time series prediction using radial basis functions, where the input data contain a significant proportion of missing points, is developed. This technique is intended to model the data while simultaneously providing a means of minimizing the impact upon the model of the missing points that are typical of geophysical time series. The two issues are inextricably entwined because missing data points have a significant impact upon the performance of data-derived models in terms of prediction availability and accuracy. The core of the technique is a nonlinear interpolation scheme that assigns values to gaps in the input time series. Each missing point is interpolated such that the error introduced into any specific predictive function is minimized. This interpolative technique has a general application in any instance where the effects of interpolation upon a given analysis process need to be minimized or a complete time series needs to be constructed from incomplete data. The technique has been applied to the prediction of fOF2 from Slough, United Kingdom. The resultant model prediction root-mean-square (RMS) error is shown to be 2.3% better than using recurrence interpolation (in terms of overall model accuracy rather than relative to each other), 3.8% better than using persistence interpolation, and 34.3% better than not using any interpolation. Utilizing the interpolation algorithm lowers the RMS error by 26% when incomplete data, in addition to complete data, are used as an input to both the interpolated and the uninterpolated models.

In New Zealand after World War 2 radar techniques were used in various investigations in geophysics and astronomy. Much local expertise had become available from defence laboratories, which had been set up in 1939 and eventually merged into the Radio Development Laboratory, disbanded in 1946. Wartime radar development had in turn been founded on pre-war research in radio propagation and ionospheric research which included the use of pulse ionosondes. Frequent support for the pre-war radio research in both Britain and New Zealand was given by Ernest Rutherford who, throughout his life, retained the interest from his own early researches in radio wave propagation. This paper is a brief survey of events from Rutherford's early experiments in 1894 to present-day research programmes.

An empirical model for F-region peak ionospheric storm-time changes has been developed based on understanding from theoretical modeling of geomagnetic storms. The model is designed to scale climatology, or monthly-medians, based on the strength of a storm, as a function of geomagnetic latitude, season, and local time. The model is driven by an index derived from the previous thirty hours of auroral or geomagnetic activity, suitably weighted by a filter. The model is particularly effective in capturing the ionospheric storm negative phase in summer mid latitudes, where it reduces the root-mean-square error by more than a factor of two. The winter mid-latitude F region typically experiences a positive phase during a storm accompanied by a high degree of variability. The model does less well in these circumstances but still makes a significant reduction in the variance. The ionospheric storm-time model can be used to scale monthly median values or a quiet time model such as the International Reference Ionosphere.

Dual frequency Global Positioning System (GPS) receivers provide integrated total electron content (TEC) along the ray path (slant TEC, affected by a bias). By inverting this observable, it is possible to obtain the vertical total electron content with some assumptions about the horizontal structure of the ionosphere. The large number of permanent receivers distributed around the world provide enough information to obtain such TEC observables with high spatial and temporal resolutions. Nevertheless, the geometry (mainly vertical) of the ground GPS observations does not allow to solve the vertical structure of electron density of the ionosphere. Mixing different kinds of complementary data in a tomographic context helps to overcome this problem. Several works have obtained successful results achieved by combining occultation and ground GPS data to estimate the local three-dimensional structure of ionospheric electron density. This paper proposes the use of just ground data to obtain similar or better results. To do this, the ground GPS data are mixed with vertical profiles of electron density derived from ionosonde data instead of GPS occultation observations. In this paper, the complementarity between vertical profiles of electron density (estimated using the NeQuick model) and ground GPS data (from GPS IGS permanent network) are shown as well as the performance of the resulting combination.

The October 2003 geomagnetic storm (often called the Halloween storm) was one of the largest storms (as measured by Dst) yet recorded. The storm-induced synoptic-scale changes in the ionosphere's plasma content and density can be viewed through space weather maps created by objective analysis algorithms. For this study, these maps, which specify the electron density in altitude, latitude, and longitude, are created by the ionospheric data assimilation three dimensional (IDA3D), a three-dimensional variation algorithm of the ionospheric electron density. These maps, representing the average conditions in the ionosphere over a 15 min sampling time, show how dramatically the ionosphere changed during the Halloween storm. Following the southward turning of the interplanetary magnetic field, the dayside electron content is significantly reduced in the equatorial ionosphere between ±18^o magnetic latitude and is enhanced poleward of this latitude. This is the expected behavior when the equatorial fountain is enhanced by a strong penetration electric field. In addition, the electron content is significantly increased in the dayside midlatitude ionosphere, which corresponds to a storm-enhanced density (SED) plume. Above 40^o magnetic latitude, the dayside plasma content is significantly reduced in the regions adjacent to the SED structure, which enhances the electron content gradient. Electron density maps in the altitude-magnetic latitude plane show an increase in the topside electron densities within an SED plume.

Data from midlatitude ionograms scaled by the computer system, ARTIST, are compared with data from the standard manual method. Differences between the scaled values for foF2 and M(3000)F2 are presented for five periods of low sunspot activity between 1984 and 1986. It is found that the ARTIST system provides acceptable data about 93 pct of the time. The system does not perform as well in summer due to the presence of blanketing-type Es and the proximity of foF2 to foF1.

The climatological behaviour of the thermospheric meridional wind above Kiruna, Sweden (67.4^oN, 20.4^oE) has been investigated for seasonal and solar cycle dependence using six different techniques, comprising both model and experimental sources. Model output from both the empirical Horizontal Wind Model (HWM) (Hedin et al., 1988) and the numerical Coupled Thermosphere and Ionosphere Model (CTIM) are compared to the measured behaviour at Kiruna, as a single site example. The empirical International Reference Ionosphere (IRI) model is used as input to an implementation of servo theory, to provide another climatology combining empirical input with a theoretical framework. The experimental techniques have been introduced in a companion paper in this issue and provide climatologies from direct measurements, using Fabry-Perot Interferometers (FPI), together with 2 separate techniques applied to the European Incoherent Scatter radar (EISCAT) database to derive neutral winds. One of these techniques uses the same implementation of servo theory as has been used with the IRI model. Detailed comparisons for each season and solar activity category allow for conclusions to be drawn as to the major influences on the climatological behaviour of the wind at this latitude. Comparison of the incoherent scatter radar (ISR) derived neutral winds with FPI, empirical model and numerical model winds is important to our understanding and judgement of the validity of the techniques used to derive thermospheric wind databases. The comparisons also test model performance and indicate possible reasons for differences found between the models. In turn, the conclusions point to possible improvements in their formulation. In particular it is found that the empirical models are over-reliant on mid-latitude data in their formulation, and fail to provide accurate estimates of the winds at high-latitudes.

Spatial maps of the ionosphere-plasmasphere slab thickness (T) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1 degrees spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35 degrees to 70 degrees N, and longitudes -10 degrees to 40 degrees E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS-TEC and ionosonde observations at solar minimum (1995-1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor (omega) is introduced as a ratio of disturbance to quietness representing area mean TEC,foF2 and tau for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and T maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but T enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main,phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate-the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS-T.EC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300km by night.

In December 1990 a new IRI handbook was published by NASA's National Space Science Data Center (NSSDC) describing in detail the International Reference Ionosphere 1990. Shortly thereafter, the IRI-90 software was released on tape, diskette, and computer networks. This paper is intended as an inventory of the most important IRI activities up to 1990 and as a starting point for the next improvement cycle. It summarizes the work and studies that led to IRI-90 and provides an overview over this latest version of the model. Shortcomings and limitations are pointed out and ways of overcoming them are discussed. Priorities are suggested for the list of work items that the IRI group has to tackle in the future. High on the wish-list are (i) major improvements at high latitudes and (ii) inclusion of magnetic storm effects. This paper deals with plasma temperatures, ion composition, and ion drift; the preceding companion paper discusses the electron density.

Data from 23 ionospheric stations are used for September 1999 to produce the electron temperature, Te, at the F2 layer peak height, hmF2, on the base of empirical relation between Te and the electron density, Ne, at a given height for a given index of solar radio flux (Brace and Theis, 1978, 1984). Daily and monthly Te minimum, mean, and maximum are then evaluated for each station. Near the solar maximum monthly Te(min) is about 1470K while the monthly Te(max), occurring during sunrise, varies with location in a wide range from 1900 to 3900K. A new weighted scheme is suggested for forecast of magnetic activity 3 h in advance by accumulation of selected magnetic indices ranked by decreasing order for 12 hrs preceding given time of observation. By using the daily Te values, cold and hot ionospheric days during the month have been defined: the magnetic activity does not influence Te at low and equatorial latitudes while at mid-latitudes a high degree of correlation is found between the F2 peak plasma heating and weighted accumulation of the magnetic indices. The most important effect of Te heating at sunrise is observed at high latitudes during the recovery phase of magnetic storm and sub-storm.

It has been shown that the conventional threshold of +/-20% departures from monthly median cannot serve for reliably distinguishing quiet and disturbed ionospheric conditions at different latitudes/time-of-day/season/level of solar activity. After a 3 h filtering of daily-hourly foF2 critical frequency, for each 3 h UT bin new upper and lower variability boundaries are introduced, based on the extreme foF2 values normalized to the monthly median similar to assessments of warming-cooling of air temperature in meteorology. Application of so defined boundaries is made to long-term observations at 56 ionospheric stations world-wide for the period of 1942 to 1999 comprising in total more than 13,000,000 hourly foF2 values.

A survey is presented of hourly averages of observations of the interplanetary medium, made by satellites close to the Earth (i.e. at 1 a.u.) in the years 1963-1986. This survey therefore covers two complete solar cycles (numbers 20 and 21). The distributions and solar-cycle variations of IMF field strength, B, and its northward component (in GSM coordinates), B_z, and of the solar-wind density, n, speed, υ, and dynamic pressure, P, are discussed. Because of their importance to the terrestrial magnetosphere/ionosphere, particular attention is given to B_z and P. The solar-cycle variation in the magnitude and variability of B_z, previously reported for cycle 20, is also found for cycle 21. However, the solar-wind data show a number of differences between cycles 20 and 21. The average dynamic pressure is found to show a solar-cycle variation and a systematic increase over the period of the survey. The minimum of dynamic pressure at sunspot maximum is mainly due to reduced solar-wind densities in cycle 20, but lower solar-wind speed in cycle 21 is a more significant factor. The distribution of the duration of periods of stable polarity of the IMF B_z component shows that the magnetosphere could achieve steady state for only a small fraction of the time and there is some evidence for a solar-cycle variation in this fraction. It is also found that the polarity changes in the IMF B_z fall into two classes: one with an associated change in solar-wind dynamic pressure, the other without such a change. However, in only 20% of cases does the dynamic pressure change exceed 50%.

The study of historical great geomagnetic storms is crucial for assessing the possible risks to the technological infrastructure of a modern society, caused by extreme space-weather events. The normal benchmark has been the great geomagnetic storm of 1859 September, the so-called ”Carrington Event.” However, there are numerous records of another great geomagnetic storm in 1872 February. This storm, which occurred about 12 years after the Carrington Event, resulted in comparable magnetic disturbances and auroral displays over large areas of the Earth. We have revisited this great geomagnetic storm in terms of the auroral and sunspot records in historical documents from East Asia. In particular, we have surveyed the auroral records from East Asia and estimated the equatorward boundary of the auroral oval to be near 24.2^o invariant latitude, on the basis that the aurora was seen near the zenith at Shanghai (20^o magnetic latitude, MLAT). These results confirm that this geomagnetic storm of 1872 February was as extreme as the Carrington Event, at least in terms of the equatorward motion of the auroral oval. Indeed, our results support the interpretation of the simultaneous auroral observations made at Bombay (10^o MLAT). The East Asian auroral records have indicated extreme brightness, suggesting unusual precipitation of high-intensity, low-energy electrons during this geomagnetic storm. We have compared the duration of the East Asian auroral displays with magnetic observations in Bombay and found that the auroral displays occurred in the initial phase, main phase, and early recovery phase of the magnetic storm.

Inversion algorithms are available to derive the vertical electron density profile at the midpoint of an oblique sounder path. The techniques open up the possibility of monitoring the ionosphere at otherwise inaccessible locations, such as over sea or inhospitable terrain. A new method of monitoring the ionosphere based on radio tomography can be used to create two-dimensional images of electron density. The results in this paper compare midpoint profiles derived from oblique ionograms with corresponding profiles obtained from tomographic images of electron density and from a vertical ionospheric sounder. The comparisons illustrate the oblique sounder inversion technique and its inherent limitations. The results provide useful information on the complementary nature of the separate ionospheric measurement techniques and have implications for the use of these measurements as inputs to real-time ionospheric models.

GPS radio occultations allow the sounding of the Earth's atmosphere (i.e. troposphere and ionosphere). The basic observable of this technique is the additional delay, due to the refractivity index, of a radio signal when passing through the atmosphere. This additional delay is proportional to the integrated refractivity, in such a way that we can obtain an estimation of the vertical refractivity profiles using observations at different elevation angles by solving an inverse problem. Traditionally, the solution of this inverse problem is obtained by using the Abel inversion algorithm assuming a refractivity index that only depends on the altitude. In this paper we present a modified Abel inversion algorithm for ionospheric sounding that overcomes the spherical symmetry assumption of the traditional Abel inversion algorithm. Processing a set of simulated data and 1 day of real data with this algorithm, a clear improvement over the traditional one can be obtained when comparing the derived critical frequencies with the ionosonde measurements. It is also shown that this improvement is sufficient to measure critical frequencies associated with the ionospheric E layer.

Based on a 37-year-long (1958-1994) series of hourly measurements of the ionospheric E-region critical frequency f_oE at four stations (Moscow, Kaliningrad, Slough, and Boulder), we determine the ionization index I_E (the fourth power of the normalized critical frequency) and analyze its correlation with solar radio flux F_10.7 during maxima and minima of 27-day variations in F_10.7 The coefficients of the linear regression equation that describes the correlation of I_E with F_10.7 have been found to differ markedly during these periods and exhibit semiannual variations. Possible causes of these effects are discussed.

Published estimates of the trend in hmF2 using data from ionosondes over the last 30-40 years range from +0.8 to -0.6 km yr^-1 and are subject to the influence of several factors. These are considered here based upon an analysis of two southern hemisphere geomagnetically mid-latitude stations, Argentine Islands and Port Stanley. The influence of the equation used to calculate hmF2 at these stations can result in variations of ±0.2 km yr^-1; choice of solar proxy has a small influence on the end result, where using E10.7 instead of F10.7 produces changes of -0.04 km yr^-1; neglecting any trends in geomagnetic activity can produce variations of +0.03 to +0.2 km yr^-1 at the two mid-latitude stations considered in this paper; for datasets of 30-40 years length ringing due to long memory processes can produce ±0.2 km yr^-1 variability; the phase of the 11-year solar cycle, and its harmonics, captured by the datasets can cause variability of ±0.5 km yr^-1; and the neglect of local time variations in thermospheric wind conditions could result in +0.2 km yr^-1 for analysis which only considers local midday data. The Argentine Islands and Port Stanley datasets show ringing terms that are still converging towards trend results of -0.25 to -0.30 km yr^-1, which are in close agreement with the satellite drag trend estimates.

F-region peak heights, derived from ionospheric scaled parameters through 38-year data series from both Argentine Islands (65 S, 64 W) and Port Stanley (52 S, 58 W) have been analysed for signatures of secular change. Long-term changes in altitude, which vary with month and time of day, were found at both sites. The results can be interpreted either as a constant decrease in altitude combined with a decreasing thermospheric wind effect, or a constant decrease in altitude which is altitude-dependent. Both interpretations leave inconsistencies when the results from the two sites are compared. The estimated long-term decrease in altitude is of a similar order of magnitude to that which has been predicted to result in the thermosphere from anthropogenic change related to greenhouse gases. Other possibilities should not, however, be ruled out.

A time-weighted accumulation of the ap index, ap(small tau, Greek) (Wrenn, 1987; Wrenn et al., 1987, 1989), together with other similar indices, was explored as a predictor of ionospheric behaviour, using f_oF2 data for a selection of locations in Australia and Europe for September and October 1989. All the time accumulated indices showed improved linear correlations, indicative of a response time of the order of about 15 hours. The response time could be decomposed into a lag between respective time series and a persistence time, although the decomposition appeared unnecessary as the persistence time carried the same information. Of the individual indices investigated, aa(small tau, Greek) appeared best and the auroral oval equatorward edge index (AI index) was poorest, although the differences were not statistically significant. Comparisons between the aa, ap and Kp indices, plus comparisons between different ionospheric parameters showed that forecasting may be improved using different transformations of the data. While these results appear good, further studies using other stations and seasons are warranted to confirm their utility for forecasting.

We present a study of the geographic location of lightning affecting the ionospheric sporadic-E (Es) layer over the ionospheric monitoring station at Chilton, UK. Data from the UK Met Office's Arrival Time Difference (ATD) lightning detection system were used to locate lightning strokes in the vicinity of the ionospheric monitoring station. A superposed epoch study of this data has previously revealed an enhancement in the Es layer caused by lightning within 200km of Chilton. In the current paper, we use the same data to investigate the location of the lightning strokes which have the largest effect on the Es layer above Chilton. We find that there are several locations where the effect of lightning on the ionosphere is most significant statistically, each producing different ionospheric responses. We interpret this as evidence that there is more than one mechanism combining to produce the previously observed enhancement in the ionosphere.

In previous attempts to detect eclipse-induced AGW, it has always been difficult to establish a direct link between individual waves and a specific source. This study reports observations of travelling ionospheric disturbances made in the UK at the time of the total solar eclipse of 11 August 1999. The speed and direction of the waves were estimated by a four-station array using the HF Doppler technique. In addition, the wave observations were supported by two other propagation paths, one in the north of England close to the main array and the other further afield, between the UK and Sweden. The AGW activity following the eclipse totality was different to the background waves detected before this time in amplitude, speed and direction. The velocity vectors are consistent with a generating mechanism for the waves based on the supersonic passage of the cooled region of the atmosphere during the eclipse.

The vertical sounding data at chains of ionosphere stations are used to obtain relative variations of electron concentration in the F2 ionosphere region. Specific isolated traveling large-scale irregularities are distinguished in the diurnal succession of the f_oF2 relative variations records. The temporal shifts of the irregularities at the station chains determine their motion velocity (of the order of speed of sound) and spatial scale (of order of 3000-5000 kin, the trajectory length being up to 10000 km). The motion trajectories of large-scale isolated irregularities which had preceded the earthquakes are reconstructed.

In this paper we report coordinated multispacecraft and ground-based observations of a double substorm onset close to Scandinavia on November 17, 1996. The Wind and the Geotail spacecraft, which were located in the solar wind and the subsolar magnetosheath, respectively, recorded two periods of southward directed interplanetary magnetic field (IMF). These periods were separated by a short northward IMF excursion associated with a solar wind pressure pulse, which compressed the magnetosphere to such a degree that Geotail for a short period was located outside the bow shock. The first period of southward IMF initiated a substorm growth phase, which was clearly detected by an array of ground-based instrumentation and by Interball in the northern tail lobe. A first substorm onset occurred in close relation to the solar wind pressure pulse impinging on the magnetopause and almost simultaneously with the northward turning of the IMF. However, this substorm did not fully develop. In clear association with the expansion of the magnetosphere at the end of the pressure pulse, the auroral expansion was stopped, and the northern sky cleared. We will present evidence that the change in the solar wind dynamic pressure actively quenched the energy available for any further substorm expansion. Directly after this period, the magnetometer network detected signatures of a renewed substorm growth phase, which was initiated by the second southward turning of the IMF and which finally lead to a second, and this time complete, substorm intensification. We have used our multipoint observations in order to understand the solar wind control of the substorm onset and substorm quenching. The relative timings between the observations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations and consequent substorm development. Furthermore, using a relatively simple algorithm to model the tail lobe field and the total tail flux, we show that there indeed exists a close relationship between the relaxation of a solar wind pressure pulse, the reduction of the tail lobe field, and the quenching of the initial substorm.

Data from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjarvi, Finland, have been compiled to form statistics of auroral absorption based on seven years of observations. By splitting the absorption into eight magnetic local time (MLT) sectors empirical relationships between observations of precipitation and geomagnetic activity are obtained. Through the use of in-situ measurements of the solar wind (from the Wind and ACE satellites) a linear relationship between the solar wind velocity and the cosmic noise absorption in the auroral zone is also derived. A dependence on the southward IMF (Interplanetary Magnetic Field) is demonstrated with absorption increasing with successive decreases in B_z; a northward IMF appears to have little effect and neither does the eastward component, B_y.

This paper describes the development of a major space storm during November 2-11, 1993. We discuss the history of the contributing high-speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high-speed flow which prolonged the storm and the near-Earth manifestations of the storm. The 8-day storm period was unusually long; the result of a high-speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near-Earth environment was in a state of tumult. A super-dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100 F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200K within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was  190 PJ, with 30% of that generated within 24 hours of storm onset.

Results of the simulation of ionospheric parameters over American stations (Millstone Hill, Arecibo, Port Stanley, and the Argentine Islands) and the European EISCAT station are presented. The calculations have been performed with the help of the global self-consistent model of the thermosphere, ionosphere, and protonosphere (GSM TIP) for January 21, 1993. The day considered, entering into the LTCS-9 campaign period, was characterized by quiet geomagnetic conditions and moderate solar activity. It is shown that the calculated and observed values of foF2 and T_e agree satisfactorily if we take into account soft electron precipitation in the diffuse zone, located equatorward of the main auroral precipitation zone, and in the South American geomagnetic anomaly zone.

Daily values of the ionospheric characteristics foF2 and M(3000)F2 for a given hour and month are correlated with the corresponding daily values of sunspot number using measured data collected at seven European locations. The significance of applying different-order polynomials is considered and the times are confirmed when the higher-order terms are important. Mean correlation coefficients for combined data sets over all hours, months and stations are determined, together with the standard errors of estimates. Comparisons are made with corresponding figures for monthly median values derived from the same data sets.

The ionospheric record of the 1974 cyclohexane vapour cloud explosion (VCE) accident near Flixborough is re-examined in light of a new theory used to describe the acoustic field in the atmosphere and ionosphere caused by explosions on the ground. The reconstructed oblique Doppler sounding records from six radio traces agree remarkably well with experimental results when a around source explosion yield of 283±38 tons of TNT is utilized. This result, when compared to the detonation of large hydrocarbon fuel-drop-air clouds, suggests that only 14±2 tons of cyclohexane was involved in the explosion. Additionally the time of the explosion determined from the model, 15:52:08±6, agrees, within the mutual uncertainty, with that determined seismically, 15:52:15.5±2 LIT. The precision in the value of the yield and accuracy of the time of the explosion validates the model used to describe the propagation of acoustic waves by taking into account expansion, absorption, and non-linear and inhomogeneous effects in the atmosphere and ionosphere.

A method is described for combining ionosonde and riometer data which overcomes limitations in both techniques for the measurement of normal absorption in the lower ionosphere. The usefulness of these techniques is thereby considerably increased.

An empirical model is developed to describe the variations of midlatitude F region ionization along all longitudes within the dip latitude band (30^o-55^oN), induced by geomagnetic activity, by using the relative deviations (Φ) of the F region critical frequency f_oF2 from its monthly median. The geomagnetic activity is represented by the K_p index. The main statistical relationship between Φ and K_p is obtained by using 11 years of data from 26 midlatitude ionosondes. The statistical analysis reveals that the average dependence of Φ on K_p is quadratic, the average response of the ionosphere to geomagnetic forcing is delayed with a time constant T of about 18 hours, and the instantaneous distribution of Φ along local times can be assumed sinusoidal. A continuity equation is written for Φ with the "production term" being a function of Kp modulated by a sinusoidal function of local time and the "loss" term proportional to Φ with a loss coefficient β=1/T. A new, modified function of geomagnetic activity (K_f) is introduced, being proportional to Φ averaged over all longitudes. The model is defined by two standing sinusoidal waves with periods of 24 and 12 hours, rotating synchronously with the Sun, modulated by the modified function K_f. The wave amplitudes and phases, as well as their average offset, are obtained by fitting to the data. A new error estimate called "prediction efficiency" (Peff) is used, which assigns equal weights to the model errors at all deviations of data from medians. The prediction efficiency estimate gives a gain of accuracy of 29%.

The authors expand the previously developed midlatitude model, providing the relative deviation of f_oF2 from its monthly median value as a function of local time and Kp, to the global scale. To achieve this, 55 ionosonde stations, having at least 11 years of continuous data, have been selected, and the model was applied to the data from each station separately. Data from each station were grouped into 12-month bins, every bin containing all the available hourly data within the respective month of the year. The model considers the distribution of the relative deviation along the local time at any fixed moment as composed of a diurnal and a semidiurnal waves, expressed by five parameters: daily mean (average offset), diurnal and semidiurnal amplitudes and phases. The model expression is scaled by a modified function of Kp, which reflects the delayed reaction of foF2 to Kp changes. The model parameters are determined by fitting the model expression to the data in each bin. Their distribution along the geomagnetic latitude is obtained in three longitude sectors: North America-South America, Europe-Africa, and East Asia-Australia. The seasonal symmetry of model parameters in the Northern and Southern Hemispheres, which is found to be acceptable, allows the use of parameter values from both hemispheres in obtaining their latitudinal profiles. In order to produce global distribution of each of the model parameters, the respective latitudinal profiles from the three sectors were averaged and approximated by analytical expressions.

The long-term continuous increase of greenhouse gas concentration in the atmosphere and other anthropogenic influences represent serious threat for human civilization. Therefore, it is necessary to determine the long-term trends and changes in the atmosphere-ionosphere system. The observed long-term trends in the 20th century might be. however, influenced by contribution of Sun's origin, and the process of determination of anthropoger c trends from observational data may be "spoilt" by the 11-year solar cycle. The role of solar/geomagnetic activity in long-term trends in various regions of the atmosphere/ionosphere system is briefly reviewed for the first time. The ways; of avoiding or at least diminishing the effect of solar cycle on trend determination are mentioned. As for the possible solar and geomagnetic activity responsibility for part of the observed long-term trends. the two main conclusions are as follows: (i) The role of solar and geomagnetic activity in the observed long-term trends decreases with decreasing altitude from the F-region ionosphere down to the troposphere. (ii) In the 20th century the role of solar and geomagnetic activity in the observed long-term trends/changes was decreasing from its beginning towards its end.

Planetary waves are oscillations of very predominantly tropospheric origin with typical periods of about 2-30 days. Their dominant zonal wave numbers are 1, 2 and 3, i.e. the waves are of large-scale (global) character. The planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the ionosphere, including the ionospheric F2-layer. Here, we deal only with the oscillations analyzed for four European stations over a solar cycle with the use of the Meyer and Morlet wavelet transforms. Waves with periods near 5, 10 and 16 days are studied. Only events with a duration of three wave-cycles and more are considered. The 5-day period wave events display a typical duration of 4 cycles, while 10- and 16-day wave events are less persistent, with a typical duration of about 3.5 cycles and 3 cycles, respectively. The persistence pattern in terms of number of cycles and in terms of number of days is different. In terms of number of cycles, the typical persistence of oscillations decreases with increasing period. On the other hand, in terms of number of days the typical persistence evidently increases with increasing period. The spectral distribution of event duration is too broad to allow for a reasonable prediction of event duration. Thus, the predictability of the planetary wave type oscillations in foF2 seems to be very questionable.

The state of the F-region during the Energy Budget Campaign (November/December 1980, Scandinavia) is analysed from ionosonde and electron content data. Auxiliary data were taken from other ground-based measurements and from in situ measurements made during the Campaign. A description of the overall state of the F-layer from mid-latitudes to high latitudes is followed by a detailed analysis for the nights November 10/11, November 15/16 and November 30/December 1, which are of main interest for the Campaign. In higher latitudes a distinct difference was found between the geomagnetically 'quiet' period and the two 'disturbed' periods. Under 'quiet' conditions a well defined trough was observed moving equatorwards in the evening and back polewards in the morning. Under 'disturbed' conditions the latitude dependence of electron content changed drastically. Near 60 deg of geomagnetic latitude a large increase of ionization appeared which moved equatorwards during the night. The magnitude of the enhancement depended on the level of the local geomagnetic activity. The enhancment effects are attributed to the precipitation of soft electrons producing F-layer ionization in a region confined in latitude but extended in longitude.

By the Fourier series expanding method, the observed F2 layer critical frequencies (foF2) globally over 70 stations in a high solar activity year of 1958, are used to analyze the annual and semi-annual variations of foF2, and the world wide distribution features of their amplitude and phase in daytime and nighttime are studied in detail. The results for foF2 annual and semi-annual variation are summarized as follows. The midnight (2:00 LT) foF2 annual variations are noticeable in both hemispheres at mid-high latitudes, and their amplitudes are slightly larger in far pole regions than in near pole regions. Generally, at most stations, the midnight foF2 reach the maximum in summer, and no winter anomaly can be discerned. While in daytime (14:00 LT), there are pronounced annual variations with large amplitude in both hemispheres at mid-high latitudes. After carefully studying their phases, we find that these annual variations usually peak in winter, which indicate all the variations are classic winter anomaly. However, the winter anomaly is very weak in the equatorial zone and not even perceivable in South America. Moreover, the amplitude of daytime foF2 semi-annual variation is generally small in near pole regions and large in far poles region of both hemispheres. Compared with their annual component, the semi-annual variations in the tropical region are significant. Their phase distributions reveal that the semi-annual variation usually peaks in March and April. In order to explain the results mentioned above, we studied the atomic molecular ratio [O/N₂] and confirmed that the noon foF2 annual variations prevailing in mid-high latitudes are caused largely by the annual variation of [O/N₂]. As the noon foF2 semi-annual variations pronounced in far pole regions, we should consider the contribution of [O/N₂], the solar zenith angle, the solar-driven low/mid-latitude thermospheric circulation and the magnetospherically driven high-latitude circulation. Moreover, we suggest that foF2 semi-annual variations appearing in the equatorial zone are closely related to other semi-annual variations in the upper atmosphere, such as the semi-annual variation of [O/N₂], the thermospheric circulation, the geomagnetic activities and even the ionospheric electrical field.

The equivalent winds at the F layer peak are derived from global ionosonde data to investigate their solar activity variations. With increasing solar activity, the derived equivalent winds are found of nonlinearly decreased diurnal amplitudes in all seasons at most stations. This implies that the increase in ion drag more than compensates for pressure gradients and thus restrains the diurnal amplitude at high solar activity. The diurnal phase of the derived equivalent winds generally shifts later at higher solar activity. It is the first time to explicitly report this striking feature that emerged at so many stations. Another pronounced feature is that the diurnal phase has a summer-winter difference. The diurnal phases at most stations in the Northern Hemisphere are later in winter than in summer at higher solar activity. Furthermore, a decrease in the semidiurnal amplitudes of equivalent winds with increasing solar activity is evident in winter over most stations considered and in other seasons at stations with a lower dip, but the decrease trend becomes weak in other seasons at stations with a larger dip. However, complicated dependences on solar activity can be found in the diurnal mean and the semidiurnal phases of equivalent winds at stations considered.

Experimental data acquired by the Ionospheric Digital Database of the National Geophysical Data Center, Boulder, Colorado, from 1957 to 1990, are used to study the dependence of the G condition, F1-layer, and NmF2 negative disturbance occurrence probabilities on the solar zenith angle during summer, winter, spring, and autumn months in latitude range 1 (between -10^o and +10^o of the geomagnetic latitude, Φ), in latitude range 2 (10^o < |Φ| ≤30^o), in latitude range 3 (30^o < |φ| ≤45^o, 30^o < |Φ| ≤45^o), in latitude range 4 (45^o < |φ| ≤60^o, 45^o < |Φ| ≤60^o), and in latitude range 5 (60^o < |Φ| ≤90^o), where φ is the geographic latitude. Our calculations show that the G condition is more likely to occur during the first half of a day than during the second half of a day, at all latitudes during all seasons for the same value of the solar zenith angle. The F1-layer occurrence probability is larger in the first half of a day in comparison with that in the second half of a day for the same value of the solar zenith angle in latitude range 1 for all seasons, while the F1-layer occurrence probability is approximately the same for the same solar zenith angle before and after noon in latitude ranges 4 and 5. The F1-layer and G condition are more commonly formed near midday than close to post sunrise or pre-sunset. The chance that the daytime F1-layer and G condition will be formed is greater in summer than in winter at the given solar zenith angle in latitude ranges 2-5, while the F1-layer occurrence probability is greater in winter than in summer for any solar zenith angle in latitude range 1. The calculated occurrence probability of the NmF2 weak negative disturbances reaches its maximum and minimum values during daytime and night-time conditions, respectively, and the average night-time value of this probability is less than that by day for all seasons in all studied latitude regions. It is shown that the NmF2 normal, strong, and very strong negative disturbances are more frequent on average at night than by day in latitude ranges 1 and 2 for all seasons, reaching their maximum and minimum occurrence probability values at night and by day, respectively. This conclusion is also correct for all other studied latitude regions during winter months, except for the NmF2 normal and strong negative disturbances in latitude range 5. A difference in the dependence of the strong and very strong NmF2 negative disturbance percentage occurrences on the solar zenith angle is found between latitude ranges 1 and 2. Our results provide evidence that the daytime dependence of the G condition occurrence probability on the solar zenith angle is determined mainly by the dependence of the F1-layer occurrence probability on the solar zenith angle in the studied latitude regions for winter months, in latitude range 2 for all seasons, and in latitude ranges 4 and 5 for spring, summer, and autumn months. The solar zenith angle trend in the probability of the G condition occurrence in latitude range 3 arises in the main from the solar zenith angle trend in the F1-layer occurrence probability. The solar zenith angle trend in the probabilities of strong and very strong NmF2 negative disturbances counteracts the identified solar zenith angle trend in the probability of the G condition occurrence.

Advances in our understanding of the large-scale electric and magnetic fields in the coupled magnetosphere-ionosphere system are reviewed. The literature appearing in the period January 1991–June 1993 is sorted into 8 general areas of study. The phenomenon of substorms receives the most attention in this literature, with the location of onset being the single most discussed issue. However, if the magnetic topology in substorm phases was widely debated, less attention was paid to the relationship of convection to the substorm cycle. A significantly new consensus view of substorm expansion and recovery phases emerged, which was termed the "Kiruna Conjecture" after the conference at which it gained widespread acceptance. The second largest area of interest was dayside transient events, both near the magnetopause and the ionosphere. It became apparent that these phenomena include at least two classes of events, probably due to transient reconnection bursts and sudden solar wind dynamic pressure changes. The contribution of both types of event to convection is controversial. The realisation that induction effects decouple electric fields in the magnetosphere and ionosphere, on time scales shorter than several substorm cycles, calls for broadening of the range of measurement techniques in both the ionosphere and at the magnetopause. Several new techniques were introduced including ionospheric observations which yield reconnection rate as a function of time. The magnetospheric and ionospheric behaviour due to various quasi-steady interplanetary conditions was studied using magnetic cloud events. For northward IMF conditions, reverse convection in the polar cap was found to be predominantly a summer hemisphere phenomenon and even for extremely rare prolonged southward IMF conditions, the magnetosphere was observed to oscillate through various substorm cycles rather than forming a steady-state convection bay.

By studying energy coupling between the solar wind and the magnetosphere, Lockwood et al. (Nature, 399, 437, 1999) obtained the highest reported correlation (0.97) between interplanetary conditions and geomagnetic activity. By inverting this theory and using the 27-day recurrence of geomagnetic activity to eliminate the effect of fast flow streams, these authors were able to compute the interplanetary magnetic field from annual means of the aa geomagnetic index. Because on annual time scales the IMF obeys Parker spiral theory and using Ulysses results on the 3-dimensional structure of the heliosphere, these authors were able to compute the total coronal source flux, the open flux leaving the corona and entering the heliosphere. Test with independent interplanetary data confirmed the validity of the technique. The aa index is a homogeneous series extending back to 1868 and the results showed that the coronal source flux drifted upward throughout the last century so that its solar cycle average was a factor of 2.4 larger by its end than in 1900. This rise is confirmed by studies of isotopes deposited in ice sheets, tree rings and meteorites by the action of cosmic ray bombardment, and regression analysis with, for example ¹⁰Be abundances in ice sheets reveals that the open solar flux fell to about 25 percent of present-day values by the end of the Maunder minimum. Recent theoretic work by Solanki et al. (Nature, 408, 445, 2000) has explained this variation extremely well, in terms of the length of the solar cycle and the rate at which flux emerges through the photosphere. This therefore relates the open flux variation to magnetic phenomena in the photosphere (sunspots and faculae) that are known to modulate the total solar irradiance.

Recent studies of the variation of geomagnetic activity over the past 140 years have quantified the "coronal source" magnetic flux F_s that leaves the solar atmosphere and enters the heliosphere and have shown that it has risen, on average, by an estimated 34% since 1963 and by 140% since 1900. This variation of open solar flux has been reproduced by Solanki et al. [2000] using a model which demonstrates how the open flux accumulates and decays, depending on the rate of flux emergence in active regions and on the length of the solar cycle. We here use a new technique to evaluate solar cycle length and find that it does vary in association with the rate of change of F_s in the way predicted. The long-term variation of the rate of flux emergence is found to be very similar in form to that in F_s, which may offer a potential explanation of why F_s appears to be a useful proxy for extrapolating solar total irradiance back in time. We also find that most of the variation of cosmic ray fluxes incident on Earth is explained by the strength of the heliospheric field (quantified by F_s) and use observations of the abundance of the isotope ¹⁰Be (produced by cosmic rays and deposited in ice sheets) to study the decrease in F_s during the Maunder minimum. The interior motions at the base of the convection zone, where the solar dynamo is probably located, have recently been revealed using the helioseismology technique and found to exhibit a 1.3-year oscillation. This periodicity is here reported in observations of the interplanetary magnetic field and geomagnetic activity but is only present after 1940. When present, it shows a strong 22-year variation, peaking near the maximum of even-numbered sunspot cycles and showing minima at the peaks of odd-numbered cycles. We discuss the implications of these long-term solar and heliospheric variations for Earth's environment.

The correlation between the coronal source flux F_S and the total solar irradiance I_TS is re-evaluated in the light of an additional 5 years' data from the rising phase of solar cycle 23 and also by using cosmic ray fluxes detected at Earth. Tests on monthly averages show that the correlation with F_S deduced from the interplanetary magnetic field (correlation coefficient, r = 0.62) is highly significant (99.999%), but that there is insufficient data for the higher correlation with annual means ( r = 0.80) to be considered significant. Anti-correlations between I_TS and cosmic ray fluxes are found in monthly data for all stations and geomagnetic rigidity cut-offs ( r ranging from -0.63 to -0.74) and these have significance levels between 85% and 98%. In all cases, the fit is poorest for the earliest data (i.e., prior to 1982). Excluding these data improves the anticorrelation with cosmic rays to r = -0.93 for one-year running means. Both the interplanetary magnetic field data and the cosmic ray fluxes indicate that the total solar irradiance lags behind the open solar flux with a delay that is estimated to have an optimum value of 2.8 months (and is within the uncertainty range 0.8–8.0 months at the 90% level).

Recent paleoclimate studies provide strong evidence for an association between cosmogenic isotope production and Earth's climate throughout the holocene. These isotopes are generated by the bombardment of Earth's atmosphere by galactic cosmic rays, the fluxes of which vary in approximately inverse proportion to the total open magnetic flux of the Sun. This paper discusses how results from the Ulysses spacecraft allow us to quantify the open solar flux from observations of near-Earth interplanetary space and to study its long-term variations using the homogeneous record of geomagnetic activity. A study of the results and of their accuracy is presented. The two proposed mechanisms that could lead to the open solar flux being a good proxy for solar-induced climate change are discussed: the first is the modulation of the production of some types of cloud by the air ions produced by cosmic rays; the second is a variation in the total or spectral solar irradiance, in association with changes in the open flux. Some implications for our understanding of anthropogenic climate change are discussed.

The Ulysses spacecraft has shown that the radial component of the heliospheric magnetic field is approximately independent of latitude. This has allowed quantification of the total open solar flux from near-Earth observations of the interplanetary magnetic field. The open flux can also be estimated from photospheric magnetograms by mapping the fields up to the "coronal source surface" where the field is assumed to be radial and which is usually assumed to be at a heliocentric distance r=2.5R_S (a mean solar radius, 1R_S=6.96×10⁸ m). These two classes of open flux estimate will differ by the open flux that threads the heliospheric current sheet(s) inside Earth's orbit at 2.5R_S<r<1R₁ (where the mean Earth-Sun distance, 1R₁=1AU=1.5×10¹¹ m). We here use near-Earth measurements to estimate this flux and show that at sunspot minimum it causes only a very small ( 0.5%) systematic difference between the two types of open flux estimate, with an uncertainty that is of order ±24% in hourly values, ±16% in monthly averages, and between -6% and +2% in annual values. These fractions may be somewhat larger for sunspot maximum because of flux emerging at higher heliographic latitudes.

The abundances of cosmogenic isotopes are frequently used as an indicator of solar variability in paleoclimate studies. They reveal changes in the open magnetic flux of the Sun; however, it is not clear what mechanism is at work whereby this quantifies the effect of solar variations on our climate. Studies of how geomagnetic activity is excited by the solar wind flow have allowed quantification of this open magnetic flux of the Sun, revealing it to have more than doubled during the 20th century. The assumptions used will be analysed in the light of the second perehelion pass by the Ulysses spacecraft and shown to be valid, in that they cause uncertainties of only a few percent. This flux fills the heliosphere out to the termination shock and shields Earth from galactic cosmic rays (GCRs) and, indeed, a strong anti-correlation between GCR fluxes and the open solar flux estimates is found. This open flux is also found to have a surprisingly strong correlation with the total solar irradiance (TSI) variation caused by magnetic flux threading the solar photosphere. This correlation is shown to hold in the latest TSI data from the SoHO spacecraft. The correlations of various indictors of terrestrial climate with GCRs and TSI are very similar in their strength and significance, making distinction between potential TSI and GCR effects difficult to achieve.

Recent radar studies of field-perpendicular flows in the auroral ionosphere, in conjunction with observations of the interplanetary medium immediately upstream of the Earth's bow shock, have revealed direct control of dayside convection by the Bz component of the interplanetary magnetic field (IMF). The ionospheric flows begin to respond to both northward and southward turnings of the IMF impinging upon the magnetopause after a delay of only a few minutes in the early afternoon sector, rising to about 15 minutes nearer dawn and dusk. In both the polar cap and the auroral oval, the subsequent rise and decay times are of order 5-10 minutes. We conclude there is very little convection "flywheel" effect in the dayside polar ionosphere and that only newly-opened flux tubes impart significant momentum to the ionosphere, in a relatively narrow region immediately poleward of the cusp. These findings concerning the effects of quasi-steady reconnection have important implications for any ionospheric signatures of transient reconnection which should be considerably shorter-lived than thought hitherto. In order to demonstrate the difficulty of uniquely identifying a Flux Transfer Event (FTE) in ground-based magnetometer data, we present observations of an impulsive signature, identical with that expected for an FTE if data from only one station is studied, following an observed magnetopause compression when the IMF was purely northward. We also report new radar observations of a viscous-like interaction, consistent with an origin on the flanks of the magnetotail and contributing an estimated 15-30kV to the total cross-cap potential during quiet periods.

The plasma precipitating into the Earth's dayside auroral atmosphere has characteristics which show that it originates from the shocked solar-wind plasma of the magnetosheath. The particles of the magnetosheath plasma precipitate down a funnel-shaped region (cusp) of open field lines resulting from reconnection of the geomagnetic field with the interplanetary magnetic field. Although the cusp has long been considered a well defined spatial structure maintained by continuous reconnection, it has recently been suggested that reconnection instead may take place in a series of discontinuous events; this is the 'pulsating cusp model'. Here we present coordinated radar and satellite observations of a series of discrete, poleward-moving plasma structures that are consistent with the pulsating-cusp model.

Results from all phases of the orbits of the Ulysses spacecraft have shown that the magnitude of the radial component of the heliospheric field is approximately independent of heliographic latitude. This result allows the use of near-Earth observations to compute the total open flux of the Sun. For example, using satellite observations of the interplanetar), magnetic field, the average open solar flux was shown to have risen by 29% between 1963 and 1987 and using the aa geomagnetic index it was found to have doubled during the 20th century. It is therefore important to assess fully the accuracy of the result and to check that it applies to all phases of the solar cycle. The first perihelion pass of the Ulysses spacecraft was close to sunspot minimum, and recent data from the second perihelion pass show that the result also holds at solar maximum. The hi-h level of correlation between the open flux derived from the various methods strongly supports the Ulysses discovery that the radial field component is independent of latitude. We show here that the errors introduced into open solar flux estimates by assuming that the heliospheric field's radial component is independent of latitude are similar for the two passes and are of order 25% for daily values, failing to 5% for averaging timescales of 27 days or greater. We compare here the results of four methods for estimating the open solar flu), with results from the first and second perehelion passes by Ulysses. We find that the errors are lowest (1-5% for averages over the entire perehelion passes lasting near 320 days), for near-Earth methods, based on either interplanetary magnetic field observations or the aa geomagnetic activity index. The corresponding errors for the Solanki et al. (2000) model are of the order of 9-15% and for the PFSS method, based on solar magnetograms, are of the order of 13-47%. The model of Solanki et al. is based on the continuity equation of open flux, and uses the sunspot number to quantify the rate of open flux emergence. It predicts that the average open solar flux has been decreasing since 1987, as is observed in the variation of all the estimates of the open flux. This decline combines with the solar cycle variation to produce an open flux during the second (sunspot maximum) perihelion pass of Ulysses which is only slightly larger than that during the first (sunspot minimum) perihelion pass.

Recent reconstructions of variations in the total solar irradiance (TSI) over the last 300 years are similar in form to the variations of cosmogenic isotope abundances and the inferred variation of the open solar flux over the same interval. These reconstructions show a century-scale drift in TSI which is comparable in magnitude to the amplitude of recent solar cycle changes, namely of order 1Wm^-2. In addition, strong links between paleoclimate records and cosmogenic isotopes have been found. These results also suggest a link between open solar flux and total solar irradiance. Modelling of the evolution of emerged flux by Wang et al. (2002) reproduces the inferred large changes (by a factor of order 2) in the open solar flux on century timescales, explaining the changes in cosmic ray fluxes and hence cosmogenic isotope abundance: however, this modelling also suggests that this is not associated with significant change in the photospheric magnetic flux, which modulates TSI on 11-year timescales. Thus the changes in the open flux and cosmogenic isotopes do not appear to be linked to the 100-year drift in TSI. The long-term variation in open flux has been shown to be associated with changes in the heliographic latitude of active regions and we show that the contribution of active region faculae to the TSI has changed by 0.2Wm^-2 in the past 100 years because of the directional characteristics of the excess radiation from faculae. This suggests that small flux tubes of the "extended solar cycle", and any long-term change in their latitudes, could also have made a significant contribution to the long-term drift in TSI.

There is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

We present a detailed investigation of a magnetospheric flux transfer event (FTE) seen by the Active Magnetospheric Tracer Explorer (AMPTE) UKS and IRM satellites around 1046 UT on October 28, 1984. This event has been discussed many times previously in the literature and has been cited as support for a variety of theories of FTE formation. We make use of a model developed to reproduce ion precipitations seen in the cusp ionosphere. The analysis confirms that the FTE is well explained as a brief excursion into an open low-latitude boundary layer (LLBL), as predicted by two theories of magnetospheric FTEs, namely, that they are bulges in the open LLBL due to reconnection rate enhancements or that they are indentations of the magnetopause by magnetosheath pressure increases (but in the presence of ongoing steady reconnection). The indentation of the inner edge of the open LLBL that these two models seek to explain is found to be shallow for this event. The ion model reproduces the continuous evolution of the ion distribution function between the sheath-like population at the event center and the surrounding magnetospheric populations; it also provides an explanation of the high-pressure core of the event as comprising field lines that were reconnected considerably earlier than those that are draped over it to give the event boundary layer. The magnetopause transition parameter is used to isolate a field rotation on the boundaries of the core, which is subjected to the tangential stress balance test. The test identifies this to be a convecting structure, which is neither a rotational discontinuity (RD) nor a contact discontinuity, but could possibly be a slow shock. In addition, evidence for ion reflection off a weak RD on the magnetospheric side of this structure is found. The event structure is consistent in many ways with features predicted for the open LLBL by analytic MHD theories and by MHD and hybrid simulations. The de Hoffman-Teller velocity of the structure is significantly different from that of the magnetosheath flow, indicating that it is not an indentation caused by a high-pressure pulse in the sheath but is consistent with the motion of newly opened field lines (different from the sheath flow because of the magnetic tension force) deduced from the best fit to the ion data. However, we cannot here rule out the possibility that the sheath flow pattern has changed in the long interval between the two satellites observing the FTE and subsequently emerging into the magnetosheath; thus this test is not conclusive in this particular case. Analysis of the fitted elapsed time since reconnection shows that the core of the event was reconnected in one pulse and the event boundary layer was reconnected in a subsequent pulse. Between these two pulses is a period of very low (but nonzero) reconnection rate, which lasts about 14 mins. Thus the analysis supports, but does not definitively verify, the concept that the FTE is a partial passage into an open LLBL caused by a traveling bulge in that layer produced by a pulse in reconnection rate.

We apply a numerical model of time-dependent ionospheric convection to two directly driven reconnection pulses during a 15-min interval of southward IMF on 26 November 2000. The model requires an input magnetopause reconnection rate variation, which is here derived from the observed variation in the upstream IMF clock angle, θ. The reconnection rate is mapped to an ionospheric merging gap, the MLT extent of which is inferred from the Doppler-shifted Lyman-α emission on newly opened field lines, as observed by the FUV instrument on the IMAGE spacecraft. The model is used to reproduce a variety of features observed during this event: SuperDARN observations of the ionospheric convection pattern and transpolar voltage; FUV observations of the growth of patches of newly opened flux; FUV and in situ observations of the location of the Open-Closed field line Boundary (OCB) and a cusp ion step. We adopt a clock angle dependence of the magnetopause reconnection electric field, mapped to the ionosphere, of the form E_nosin⁴(θ/2) and estimate the peak value, E_no, by matching observed and modeled variations of both the latitude, Λ_OCB, of the dayside OCB (as inferred from the equatorward edge of cusp proton emissions seen by FUV) and the transpolar voltage Φ_PC (as derived using the mapped potential technique from SuperDARN HF radar data). This analysis also yields the time constant τ_OCB with which the open-closed boundary relaxes back toward its equilibrium configuration. For the case studied here, we find τ_OCB = 9.7 ±1.3 min, consistent with previous inferences from the observed response of ionospheric flow to southward turnings of the IMF. The analysis confirms quantitatively the concepts of ionospheric flow excitation on which the model is based and explains some otherwise anomalous features of the cusp precipitation morphology.

We study a brightening of the Lyman-a emission in the cusp which occurred in response to a short-lived south-ward turning of the interplanetary magnetic field (IMF) during a period of strongly enhanced solar wind plasma concentration. The cusp proton emission is detected using the SI-12 channel of the FUV imager on the IMAGE spacecraft. Analysis of the IMF observations recorded by the ACE and Wind spacecraft reveals that the assumption of a constant propagation lag from the upstream spacecraft to the Earth is not adequate for these high time-resolution studies. The variations of the southward IMF component observed by ACE and Wind allow for the calculation of the ACE-to-Earth lag as a function of time. Application of the derived propagation delays reveals that the intensity of the cusp emission varied systematically with the IMF clock angle, the relationship being particularly striking when the intensity is normalised to allow for the variation in the upstream solar wind proton concentration. The latitude of the cusp migrated equatorward while the lagged IMF pointed southward, confirming the lag calculation and indicating ongoing magnetopause reconnection. Dayside convection, as monitored by the SuperDARN network of radars, responded rapidly to the IMF changes but lagged behind the cusp proton emission response: this is shown to be as predicted by the model of flow excitation by Cowley and Lockwood (1992). We use the numerical cusp ion precipitation model of Lockwood and Davis (1996), along with modelled Lyman-a emission efficiency and the SI-12 instrument response, to investigate the effect of the sheath field clock angle on the acceleration of ions on crossing the dayside magnetopause. This modelling reveals that the emission commences on each reconnected field line 2-2.5 min after it is opened and peaks 3-5 min after it is opened. We discuss how comparison of the Lyman-a intensities with oxygen emissions observed simultaneously by the SI-13 channel of the FUV instrument offers an opportunity to test whether or not the clock angle dependence is consistent with the "component" or the "anti-parallel" reconnection hypothesis.

We predict the field-aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a B_Y component that is large in magnitude. For B_Y>0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for B_Y<0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field-aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field-aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF B_Y, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field-aligned current directions will be reversed. Observations by the Polar spacecraft on August 26, 1998, show the predicted upward current sheets at steps seen in the mantle region for IMF B_Y>0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field-aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.

More than 70 years ago, it was recognised that ionospheric F2-layer critical frequencies [foF2] had a strong relationship to sunspot number. Using historic datasets from the Slough and Washington ionosondes, we evaluate the best statistical fits of foF2 to sunspot numbers (at each Universal Time [UT] separately) in order to search for drifts and abrupt changes in the fit residuals over Solar Cycles 17–21. This test is carried out for the original composite of the Wolf/Zürich/International sunspot number [ R $R$ ], the new “backbone” group sunspot number [ R BB $R_{\mathrm{BB}}$ ], and the proposed “corrected sunspot number” [ R C $R_{\mathrm{C}}$ ]. Polynomial fits are made both with and without allowance for the white-light facular area, which has been reported as being associated with cycle-to-cycle changes in the sunspot-number–foF2 relationship. Over the interval studied here, R $R$ , R BB $R_{\mathrm{BB}}$ , and R C $R_{\mathrm{C}}$ largely differ in their allowance for the “Waldmeier discontinuity” around 1945 (the correction factor for which for R $R$ , R BB $R_{\mathrm{BB}}$ , and R C $R_{\mathrm{C}}$ is, respectively, zero, effectively over 20 %, and explicitly 11.6 %). It is shown that for Solar Cycles 18–21, all three sunspot data sequences perform well, but that the fit residuals are lowest and most uniform for R BB $R_{\mathrm{BB}}$ . We here use foF2 for those UTs for which R $R$ , R BB $R_{\mathrm{BB}}$ , and R C $R_{\mathrm{C}}$ all give correlations exceeding 0.99 for intervals both before and after the Waldmeier discontinuity. The error introduced by the Waldmeier discontinuity causes R $R$ to underestimate the fitted values based on the foF2 data for 1932–1945, but R BB $R_{\mathrm{BB}}$ overestimates them by almost the same factor, implying that the correction for the Waldmeier discontinuity inherent in R BB $R_{\mathrm{BB}}$ is too large by a factor of two. Fit residuals are smallest and most uniform for R C $R_{\mathrm{C}}$ , and the ionospheric data support the optimum discontinuity multiplicative correction factor derived from the independent Royal Greenwich Observatory (RGO) sunspot group data for the same interval.

The usual interpretation of a flux transfer event (FTE) at magnetopause, in terms of time-dependent and possibly patchy reconnection, demands that it generate an ionospheric signature. Recent ground-based observations have revealed that auroral transients in the cusp/cleft region have all the characteristics required of FTE effects. However, signatures in the major available dataset, namely that from low-altitude polar-orbiting satellites, have not yet been identified. In this paper, we consider a cusp pass of the DE-2 spacecraft during strongly southward IMF. The particle detectors show magnetosheath ion injection signatures. However, the satellite motion and convection are opposed, and we discuss how the observed falling energy dispersion of the precipitating ions can have arisen from a static, moving or growing source. The spatial scale of the source is typical of an FTE. A simple model of the ionospheric signature of an FTE reproduces the observed electric and magnetic field perturbations. Precipitating electrons of peak energy ∼100 eV are found to lie on the predicted boundary of the newly-opened tube, very similar to those found on the edges of FTEs at the magnetopause. The injected ions are within this boundary and their dispersion is consistent with its growth as reconnection proceeds. The reconnection potential and the potential of the induced ionospheric motion are found to be the same (≅25 kV). The scanning imager on DE-1 shows a localized transient auroral feature around DE-2 at this time, similar to the recent optical/radar observations of FTEs.

We test the method of Lockwood et al. [1999] for deriving the coronal source flux from the geomagnetic aa index and show it to be accurate to within 12% for annual means and 4.5% for averages over a sunspot cycle. Using data from four solar constant monitors during 1981–1995, we find a linear relationship between this magnetic flux and the total solar irradiance. From this correlation, we show that the 131% rise in the mean coronal source field over the interval 1901–1995 corresponds to a rise in the average total solar irradiance of ΔI = 1.65 ±0.23 Wm^-2.

The solar wind is an extended ionized gas of very high electrical conductivity, and therefore drags some magnetic flux out of the Sun to fill the heliosphere with a weak interplanetary magnetic field,. Magnetic reconnection – the merging of oppositely directed magnetic fields – between the interplanetary field and the Earth's magnetic field allows energy from the solar wind to enter the near-Earth environment. The Sun's properties, such as its luminosity, are related to its magnetic field, although the connections are still not well understood,. Moreover, changes in the heliospheric magnetic field have been linked with changes in total cloud cover over the Earth, which may influence global climate. Here we show that measurements of the near-Earth interplanetary magnetic field reveal that the total magnetic flux leaving the Sun has risen by a factor of 1.4 since 1964: surrogate measurements of the interplanetary magnetic field indicate that the increase since 1901 has been by a factor of 2.3. This increase may be related to chaotic changes in the dynamo that generates the solar magnetic field. We do not yet know quantitatively how such changes will influence the global environment.

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974-1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a "double-humped structure", which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.

On 2007 December 7, there was an eruption from AR 10977, which also hosted a sigmoid. An EUV Imaging Telescope (EIT) wave associated with this eruption was observed by EUVI on board the Solar Terrestrial Relations Observatory ( STEREO ). Using EUVI images in the 171 Å and the 195 Å passbands from both STEREO A and B , we study the morphology and kinematics of this EIT wave. In the early stages, images of the EIT wave from the two STEREO spacecrafts differ markedly. We determine that the EUV fronts observed at the very beginning of the eruption likely include some intensity contribution from the associated coronal mass ejection (CME). Additionally, our velocity measurements suggest that the EIT wave front may propagate at nearly constant velocity. Both results offer constraints on current models and understanding of EIT waves.

In this paper the origin and evolution of the Sun's open magnetic flux is considered by conducting magnetic flux transport simulations over many solar cycles. The simulations include the effects of differential rotation, meridional flow and supergranular diffusion on the radial magnetic field at the surface of the Sun as new magnetic bipoles emerge and are transported poleward. In each cycle the emergence of roughly 2100 bipoles is considered. The net open flux produced by the surface distribution is calculated by constructing potential coronal fields with a source surface from the surface distribution at regular intervals. In the simulations the net open magnetic flux closely follows the total dipole component at the source surface and evolves independently from the surface flux. The behaviour of the open flux is highly dependent on meridional flow and many observed features are reproduced by the model. However, when meridional flow is present at observed values the maximum value of the open flux occurs at cycle minimum when the polar caps it helps produce are the strongest. This is inconsistent with observations by Lockwood, Stamper and Wild (1999) and Wang, Sheeley, and Lean (2000) who find the open flux peaking 1–2 years after cycle maximum. Only in unrealistic simulations where meridional flow is much smaller than diffusion does a maximum in open flux consistent with observations occur. It is therefore deduced that there is no realistic parameter range of the flux transport variables that can produce the correct magnitude variation in open flux under the present approximations. As a result the present standard model does not contain the correct physics to describe the evolution of the Sun's open magnetic flux over an entire solar cycle. Future possible improvements in modeling are suggested.

In this paper the origin and evolution of the Sun's open magnetic flux are considered for single magnetic bipoles as they are transported across the Sun. The effects of magnetic flux transport on the radial field at the surface of the Sun are modeled numerically by developing earlier work by Wang, Sheeley, and Lean (2000). The paper considers how the initial tilt of the bipole axis (α) and its latitude of emergence affect the variation and magnitude of the surface and open magnetic flux. The amount of open magnetic flux is estimated by constructing potential coronal fields. It is found that the open flux may evolve independently from the surface field for certain ranges of the tilt angle. For a given tilt angle, the lower the latitude of emergence, the higher the magnitude of the surface and open flux at the end of the simulation. In addition, three types of behavior are found for the open flux depending on the initial tilt angle of the bipole axis. When the tilt is such that α≥2^o the open flux is independent of the surface flux and initially increases before decaying away. In contrast, for tilt angles in the range -16^o<α<2^o the open flux follows the surface flux and continually decays. Finally, for α≤-16^o the open flux first decays and then increases in magnitude towards a second maximum before decaying away. This behavior of the open flux can be explained in terms of two competing effects produced by differential rotation. Firstly, differential rotation may increase or decrease the open flux by rotating the centers of each polarity of the bipole at different rates when the axis has tilt. Secondly, it decreases the open flux by increasing the length of the polarity inversion line where flux cancellation occurs. The results suggest that, in order to reproduce a realistic model of the Sun's open magnetic flux over a solar cycle, it is important to have accurate input data on the latitude of emergence of bipoles along with the variation of their tilt angles as the cycle progresses.

On March 13, 1989 magnetic storm effects on the mid- and low-latitude ionosphere were investigated. For this, peak electron density of 172-layer (NmF2) data from four chains of ionospheric stations located in the geographic longitude ranges 10^oW-15^oE, 55^oE-85^oE, 135^oE-155^oE and 200^oE-255^oE were used. Relative deviations of perturbed NmF2 from their respective quiet-time values were considered. Long-lasting negative storm effects were the dominant characteristic observed at middle latitudes, which occurred since the main phase of the storm. In general, the most significant negative disturbances were observed at middle-high latitudes. In the longitudinal sectors in which the storm started at day-time and pre-dusk hours, positive storm effects at middle and low latitudes were observed during the main phase. The role of some physical mechanisms to explain the ionospheric effects is also considered.

The method earlier used for the foF2 long-term trends analysis is applied to reveal hmF2 long-term trends at 27 ionosonde stations in the European and Asian longitudinal sectors. Observed M(3000)F2 data for the last 3 solar cycles are used to derive hmF2 trends. The majority of the studied stations show significant hmF2 linear trends with a confidence level of at least 95% for the period after 1965, with most of these trends being positive. No systematic variation of the trend magnitude with latitude is revealed, but some longitudinal effect does take place. The proposed geomagnetic storm concept to explain hmF2 long-term trends proceeds from a natural origin of the trends rather than an artificial one related to the thermosphere cooling due to the greenhouse effect.

Keywords: Ionosphere (ionosphere-atmosphere interaction)

Ionospheric imaging usually involves solving an underdetermined inversion problem. The inversion is performed involving additional constraints to enforce realistic profiles in the vertical. One way to incorporate those vertical profile constraints is to perform the inversion using Empirical Orthogonal Functions (EOFs). The need of defining a sample space spanned by EOFs to obtain ionospheric images yields the possibility to employ ionosonde measurements in ionospheric tomography based on stochastic inversion of GPS data. Here we present a simulation study based on an existing network of GPS ground receivers and ionosondes across Europe. The locations of the transmitters used in the simulation are actual satellite positions. Simulated GPS data, constructed assuming that the ionosphere is the international reference ionosphere, are inverted via the Multi Instrument Data Analysis System. The sample space of this stochastic inversion is constructed employing ionosonde measurements simulated from the same model ionosphere. Such use of ionosonde data to construct the sample space produces better results than without ionosonde data.

Keywords: Electromagnetic methods; Ionosphere; Radio propagation; Remote sensing; Radio tomography; Data inversion

Nearly two years of 2-min resolution data and 7- to 21-s resolution data from the CUTLASS Finland HF radar have undergone Fourier analysis in order to study statistically the occurrence rates and repetition frequencies of pulsed ionospheric flows in the noon-sector high-latitude ionosphere. Pulsed ionospheric flow bursts are believed to be the ionospheric footprint of newly reconnected geomagnetic field lines, which occur during episodes of magnetic flux transfer to the terrestrial magnetosphere - flux transfer events or FTEs. The distribution of pulsed ionospheric flows were found to be well grouped in the radar field of view, and to be in the vicinity of the radar signature of the cusp footprint. Two thirds of the pulsed ionospheric flow intervals included in the statistical study occurred when the interplanetary magnetic field had a southward component, supporting the hypothesis that pulsed ionospheric flows are a reconnection-related phenomenon. The occurrence rate of the pulsed ionospheric flow fluctuation period was independent of the radar scan mode. The statistical results obtained from the radar data are compared to occurrence rates and repetition frequencies of FTEs derived from spacecraft data near the magnetopause reconnection region, and to ground-based optical measurements of poleward moving auroral forms. The distributions obtained by the various instruments in different regions of the magnetosphere were remarkably similar. The radar, therefore, appears to give an unbiased sample of magnetopause activity in its routine observations of the cusp footprint.

This paper presents results from the TIME-GCM-CCM3 thermosphere-ionosphere-lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of 'meteorological' disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year. In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density (NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N₂] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation. Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed 'meteorological' disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the 'meteorological' disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition.

All planetary atmospheres respond to the enhanced x-rays and ultraviolet (UV) light emitted from the Sun during a flare. Yet only on Earth are observations so continuous that the consequences of these essentially unpredictable events can be measured reliably. Here, we report observations of solar flares, causing up to 200% enhancements to the ionosphere of Mars, as recorded by the Mars Global Surveyor in April 2001. Modeling the attitude dependence of these effects requires that relative enhancements in the soft x-ray fluxes far exceed those in the UV.

A relationship between foE trends and geomagnetic activity long-term variations has been revealed for the first time. By analogy with earlier obtained results on the foF2 trends it is possible to speak about the geomagnetic control of the foE long-term trends as well. Periods of increasing geomagnetic activity correspond to negative foE trends, while these trends are positive for the decreasing phase of geomagnetic activity. This 'natural' relationship breaks down around 1970 (on some stations later) when pronounced positive foE trends have appeared on most of the stations considered. The dependence of foE trends on geomagnetic activity can be related with nitric oxide variations at the E-layer heights. The positive foE trends that appeared after the 'break down' effect may also be explained by the decrease which is not related to geomagnetic activity variations. But negative trends or irregular foE variations on some stations for the same time period require some different mechanism. Chemical pollution of the lower thermosphere due to the anthropogenic activity may be responsible for such abnormal foE behavior after the end of the 1960s.

The ionospheric F2-layer parameter long-term trends are considered from the geomagnetic control concept and the greenhouse hypothesis points of view. It is stressed that long-term geomagnetic activity variations are crucial for ionosphere long-term trends, as they determine the basic natural pattern of foF2 and hmF2 long-term variations. The geomagnetic activity effects should be removed from the analyzed data to obtain real trends in ionospheric parameters, but this is not usually done. Only a thermosphere cooling, which is accepted as an explanation for the neutral density decrease, cannot be reconciled with negative foF2 trends revealed for the same period. A more pronounced decrease of the O/N2 ratio is required which is not provided by empirical thermospheric models. Thermospheric cooling practically cannot be seen in foF2 trends, due to a weak NmF2 dependence on neutral temperature; therefore, foF2 trends are mainly controlled by geomagnetic activity long-term variations. Long-term hmF2 variations are also controlled by geomagnetic activity variations, as both parameters, NmF2 and hmF2 are related by the F2-layer formation mechanism. But hmF2 is very sensitive to neutral temperature changes, so strongly damped hmF2 long-term variations observed at Slough after 1972 may be considered as a direct manifestation of the thermosphere cooling. Earlier revealed negative hmF2 trends in western Europe, where magnetic declination D<0 and positive trends at the eastern stations (D>0), can be related to westward thermospheric wind whose role has been enhanced due to a competition between the thermosphere cooling (CO2 increase) and its heating under increasing geomagnetic activity after the end of the 1960s.

Further development of the method proposed by Danilov and Mikhailov is presented. The method is applied to reveal the foF2 long-term trends on 30 Northern Hemisphere ionosonde stations. Most of them show significant foF2 trends. A pronounced dependence of trend magnitude on geomagnetic (invariant) latitude is confirmed. Periods of negative/positive foF2 trends corresponding to the periods of long-term increasing/ decreasing geomagnetic activity are revealed for the first time. Pronounced diurnal variations of the foF2 trend magnitude are found. Strong positive foF2 trends in the post-midnight-early-morning LT sector and strong negative trends during daytime hours are found on the sub-auroral stations for the period with increasing geomagnetic activity. On the contrary middle and lower latitude stations demonstrate negative trends in the early-morning LT sector and small negative or positive trends during daytime hours for the same period. All the morphological features revealed of the foF2 trends may be explained in the framework of contemporary F2-region storm mechanisms. This newly proposed F2-layer geomagnetic storm concept casts serious doubts on the hypothesis relating the F2-layer parameter long-term trends to the thermosphere cooling due to the greenhouse effect.

Earlier revealed morphological features of the foF2 and hmF2 long-term trends are interpreted in the scope of the geomagnetic control concept based on the contemporary F2-layer storm mechanisms. The F2-layer parameter trends strongly depend on the long-term varying geomagnetic activity whose effects cannot be removed from the trends using conventional indices of geomagnetic activity. Therefore, any interpretation of the foF2 and hmF2 trends should consider the geomagnetic effects as an inalienable part of the trend analysis. Periods with negative and positive foF2 and hmF2 trends correspond to the periods of increasing or decreasing geomagnetic activity with the turning points around 1955, and the end of 1960s and 1980s, where foF2 and hmF2 trends change their signs. Such variations can be explained by neutral composition, as well as temperature and thermospheric wind changes related to geomagnetic activity variations. In particular, for the period of increasing geomagnetic activity (1965-1991) positive at lower latitudes, but negative at middle and high latitudes, foF2 trends may be explained by neutral composition and temperature changes, while soft electron precipitation determines nighttime trends at sub-auroral and auroral latitudes. A pronounced dependence of the foF2 trends on geomagnetic (invariant) latitude and the absence of any latitudinal dependence for the hmF2 trends are due to different dependencies of NmF2 and hmF2 on main aeronomic parameters. All of the revealed latitudinal and diurnal foF2 and hmF2 trend variations may be explained in the frame-work of contemporary F2-region storm mechanisms. The newly proposed geomagnetic storm concept used to explain F2-layer parameter long-term trends proceeds from a natural origin of the trends rather than an artificial one, related to the thermosphere cooling due to the greenhouse effect. Within this concept, instead of cooling, one should expect the thermosphere heating for the period of increasing geomagnetic activity (1965-1991).

A new approach to extract foF2 long-term trends, which are free to a great extent from solar and geomagnetic activity effects, has been proposed. These trends are insensitive to the phase (increasing/decreasing) of geomagnetic activity, with long-term variations being small and insignificant for such relatively short time periods. A small but significant residual foF2 trend, with the slope K_r = - 2.2 × 10^-4 per year, was obtained over a 55-year period (the longest avail-able) of observations at Slough. Such small trends have no practical importance. On the other hand, negative (although insignificant) residual trends obtained at 10 ionosonde stations for shorter periods (31 years) may be considered as a manifestation of a very long-term geomagnetic activity increase which did take place during the 20th century. All of the revealed foF2 long-term variations (trends) are shown to have a natural origin related to long-term variations in solar and geomagnetic activity. There is no indication of any manmade foF2 trends.

The monthly mean relative sunspot number (R_M) is assumed to contain a component (R_v) which has a one-to-one correlation with the critical frequency of the F₂-layer in an undisturbed ionosphere and which is, therefore, an idealized index of solar activity. The residual component (R_x) may be regarded as an error which has a Standard Deviation of about 20 per cent. A new index (I_F2) has been constructed for the period 1938–1954; like R_M, it can also be regarded as giving an approximate value of R_v, but its residual error component (R_z) has an S.D. which is only about one tenth that of R_x. The magnitude of I_F2, for a given month is computed from the mean noon critical frequencies in the F₂-layer at Slough, Huancayo, and Watheroo, which are normally available within a few weeks of the end of each month. The index is based, in effect, on a calibration of the F₂-layer critical frequencies at these observatories in terms of R_v, using data extending back as far as possible. Precautions have been taken to reduce to negligible proportions the effects of ionospheric disturbances on the magnitude of the new index.

A monthly index has been constructed, for the period 1938 to date, using monthly mean or median noon values of foF2 at eleven widely-distributed stations. The correlation between foF2 at noon and this index is significantly greater than that between foF2 and either the 3 month weighted mean sunspot number or the monthly mean solar radio noise flux at 2800 Mc/s. Numerical estimates have been made of the errors incurred in forecasting noon and midnight foF2 several months ahead using these three indices as guides to the trend of solar activity.

Using a numerical implementation of the cowlock92 model of flow excitation in the magnetosphere-ionosphere (MI) system, we show that both an expanding (on a  12-min timescale) and a quasi-instantaneous response in ionospheric convection to the onset of magnetopause reconnection can be accommodated by the Cowley-Lockwood conceptual framework. This model has a key feature of time dependence, necessarily considering the history of the coupled MI system. We show that a residual flow, driven by prior magnetopause reconnection, can produce a quasi-instantaneous global ionospheric convection response; perturbations from an equilibrium state may also be present from tail reconnection, which will superpose constructively to give a similar effect. On the other hand, when the MI system is relatively free of pre-existing flow, we can most clearly see the expanding nature of the response. As the open-closed field line boundary will frequently be in motion from such prior reconnection (both at the dayside magnetopause and in the cross-tail current sheet), it is expected that there will usually be some level of combined response to dayside reconnection.

A new technique is developed for forecasting and instantaneous mapping of the ionospheric parameters over Europe, based on analytical presentation of the mapped quantities. The diurnal and seasonal variations of the ionospheric foF2 and M(3000)F2 parameters are represented by a modified version of the regional model ISIRM adjusted to the past measured data. An autoregressive extrapolation of the data from the past month enables the 15-day-ahead forecast of the quiet ionospheric distribution to be performed. In addition, the short-term variations due to geomagnetic activity are defined as a plane surface superimposed on the quiet distribution. This correction is obtained by two plane characteristics as functions of the geomagnetic three-hour Kp index. In this way the 24-hour forecast can be obtain during quiet as well as disturbed ionospheric conditions. The corresponding EIFM software provides a variety of options to perform the short-term forecast depending on availability of the measured ionospheric data and predicted Kp values.

A new method for short-term prediction of ionospheric parameters is developed by incorporating the cross-correlation between the ionospheric characteristic of interest and the A_p index into the autocorrelation analysis. We consider the hourly time series of an ionospheric characteristic as composed of a periodic component and a random component. The periodic component containing the average diurnal variation is removed by using its relative deviations from the median values (Φ), which in the case of the critical frequency of the F2 layer, foF2, has the form: Φ= (foF2 - foF2_med)/foF2_med. The geomagnetically correlated autoregression model (GCAM) is an extrapolation model based on the weighted past data. The new term in the regression equation expresses linearly the dependence of Φ on magnetic activity by introducing a synthetic geomagnetic index G, which approximates the average dependence of on hourly interpolated K_p. Using parametric expressions of the auto- and cross-correlation functions ensures the statistical sufficiency in GCAM; the parameters are then obtained by data fitting. Data from 2 years of high solar activity (1981-2) and 2 years of low solar activity (1985-6) were used to evaluate the prediction accuracy of GCAM. The mean square error in per cent of the 1-day prediction of foF2 relative to the median shows a large gain of accuracy of GCAM in the first 8-10 h of prediction relative to the median based prediction, a diurnal variation of errors and a steady offset of the GCAM prediction error from the median based prediction error. The GCAM error at the first hour is lowest, but gradually approaches the median error with a timescale of 8-10 h. A new error estimate, called `prediction efficiency' that is a good indicator of prediction performance during disturbed ionospheric conditions is defined.

This paper is the second in a series on a study of the link between IMF and sporadic-E layers within the polar cap. In Paper I (Voiculescu et al., 2006), an analysis of the sporadic-E data from Thule and Longyearbyen was presented. Here we concentrate on the electric field mechanism of sporadic-E generation. By means of model calculations we show that the mechanism is effective even at Thule, where the direction of the geomagnetic field departs from vertical only by 4. The model calculations also lead to a revision of the electric field theory. Previously, a thin layer was assumed to grow at a convergent null in the vertical ion velocity, which is formed when the electric field points in the NW sector. Our calculations indicate that in the dynamic process of vertical plasma compression, a layer is generated at altitudes of high vertical convergence rather than at a null. Consequently, the layer generation is less sensitive than previously assumed to fluctuations of the electric field direction within the NW sector. The observed diurnal variations of sporadic-E occurrence at Longyearbyen and Thule are compared with the diurnal variations of the electric field, calculated using a representative range of IMF values by means of the statistical APL model. The results indicate that the main features of Es occurrence can be explained by the convection pattern controlled by the IMF. Electric fields calculated from the IMF observations are also used for producing distributions of sporadic-E occurrence as a function of electric field direction at the two sites. A marked difference between the distributions at Thule and Longyearbyen is found. A model estimate of the occurrence probability as a function of electric field direction is developed and a reasonable agreement between the model and the experimental occurrence is found. The calculation explains the differences between the distributions at the two sites in terms of the polar cap convection pattern. The conclusion is that the electric field is the major cause for sporadic-E generation and, consequently, IMF has a clear control on the occurrence of sporadic E within the polar cap.

The meridional propagation velocities of the ionospheric F2-region response to 268 geomagnetic storms are calculated. Ionospheric vertical sounding data of I h time resolution from several stations located in a longitude sector approximately centred along the great circle that contains both the geomagnetic poles and the geographic poles are used. Most meridional propagation velocities from high to low latitudes are less than 600 m/s. The smaller velocities are typical of global neutral meridional wind circulation and the larger are representative of traveling atmospheric disturbances. Simultaneous disturbances at several locations are more frequent during positive phases than during negative phases. Negative phase meridional propagation velocities associated with meridional neutral winds are less frequent in the southern hemisphere when compared with corresponding velocities observed in the northern hemisphere. This may be related to the fact that the distance between the geomagnetic pole and the equator is smaller in the northern hemisphere. Most negative phase onsets are within the 06-10 LT interval. For middle geomagnetic latitudes a "forbidden time interval" between 11 and 14 LT is present. The positive phase onsets show the "dusk effect".

The Polar spacecraft had a prolonged encounter with the high-latitude dayside magnetopause on May 29, 1996. This encounter with the magnetopause occurred when the interplanetary magnetic field was directed northward. From the three-dimensional electron and ion distribution functions measured by the Hydra instrument, it has been possible to identify nearly all of the distinct boundary layer regions associated with high-latitude reconnection. The regions that have been identified are (1) the cusp; (2) the magnetopause current layer; (3) magnetosheath field lines that have interconnected in only the Northern Hemisphere; (4) magnetosheath field lines that have interconnected in only the Southern Hemisphere; (5) magnetosheath field lines that have interconnected in both the Northern and Southern Hemispheres; (6) magnetosheath that is disconnected from the terrestrial magnetic field; and (7) high-latitude plasma sheet field lines that are participating in magnetosheath reconnection. Reconnection over this time period was occurring at high latitudes over a broad local-time extent, interconnecting the magnetosheath and lobe and/or plasma sheet field lines in both the Northern and Southern Hemispheres. Newly closed boundary layer field lines were observed as reconnection occurred first at high latitudes in one hemisphere and then later in the other. These observations establish the location of magnetopause reconnection during these northward interplanetary magnetic field conditions as being at high latitudes, poleward of the cusp, and further reinforce the general interpretation of electron and ion phase space density signatures as indicators of magnetic reconnection and boundary layer formation.

The intense level of solar activity recorded from 16 to 23 January 2005 led to a series of events with different signatures at the Earth's ionospheric distances. Measurements of the critical frequency of the F2 layer foF2 and the vertical total electron content (VTEC) are used to describe the temporal and spatial electron density distributions during this space weather event, which gives an excellent opportunity to test regional VTEC maps over Europe under such disturbed solar-terrestrial conditions. In this context, the tests used to validate the International GNSS Service (IGS) VTEC maps have been applied to assess the accuracy of the European Rutherford Appleton Laboratory (RAL) VTEC maps. Thus the self-consistency test and the Jason altimeter test have been used to compare such performances with the IGS and Universitat Politecnica de Catalunya global ionospheric maps. The results show discrepancies between the RAL maps and the IGS ones, which leads to significant RMS and bias values of several total electron content units. Moreover, in this work a kriging technique to improve the accuracy of any regional VTEC map is also considered, with relative improvements of the RAL VTEC maps up to more than 20% at the peak of the storm.

In this paper, the variations of the magnetic field, due to current flowing in the ionospheric E-region over Elazig-Turkey during the August 11, 1999 total solar eclipse have been computed. It is shown that the solar eclipse has no significant effects on the north-south component of the magnetic field. However, the westward component of the magnetic field is decreased by the solar eclipse. Therefore, we would expect that the change in the magnetic field in E-region during the solar eclipse would modify the geomagnetic field at ground level.

For solar cycle 21 (1976 - 1986) the variation of monthly mean values of noon-time foF2 at Slough, Rome, and Manila are examined by using solar flare index and geomagnetic Ap index. A single regression analysis for dependence of foF2 on solar flare index shows better matching. Moreover, less hysteresis effect is seen when we use solar flare index instead of other solar indices. Thus, for making prediction, one needs to take into account just the solar flare index and not the solar flare index and geomagnetic Ap index simultaneously.

An instantaneous space-weighted ionospheric regional model (ISWIRM) for the regional now-casting of the critical frequency of the F2 layer ( foF2) has been developed. The geographical area of applicability of the model is ranged between 35^oN - 70^oN and 5^oW - 40^oE. Inside this region the hourly values of foF2 are obtained, correcting the monthly medians values of foF2 predicted by the space-weighted ionospheric local model ( SWILM) on the basis of hourly observations of foF2 coming from four reference stations ( Rome, Chilton, Lycksele, and Loparskaya ( or Sodankyla)). The performance of the model, evaluated at four testing stations ( Tortosa, Juliusruh, Uppsala, and Kiruna) during some periods characterized by strong solar and geomagnetic activity, can be considered satisfactory, given that the hourly values of the residuals are almost always below 1 MHz. A comparison between ISWIRM's performance using manually validated and autoscaled data of foF2 and SWILM's performance was made for two disturbed periods. One example of instantaneous ionospheric mapping of foF2 relative to the selected disturbed periods is also shown.

Based on data from a network of ionospheric stations located in the range of geographic longitudes 19^o-285^o and invariant latitudes 53^o-70^oN we have investigated diurnal behaviour variations in F2-layer critical frequencies for different seasons and different levels of solar activity. The study revealed that the longitudinal effect in diurnal foF2 variations is most conspicuous in the summertime at the invariant latitude about 55-57^oN and manifests itself in the shift of the foF2 maximum into the evening and night-time hours on the Yakutsk (129.6^o) and Ottawa (284^o) meridians in the region of westward declination. It is likely that such a behaviour of foF2 is conditioned by a change in the dynamic regime of the high-latitude ionosphere associated with the magnetic anomaly.

We report the results of our investigations on the potential ionospheric effects caused by the 15 February 2013 Chelyabinsk meteor explosion. We used the data from a number of digisonde stations located in Europe and Russia to detect the traveling ionospheric disturbances (TIDs) likely to have been caused by the meteor explosion. We found that certain characteristic signatures of the TIDs can be identified in individual ionogram records, mostly in the form of Y-forking/splitting of the ionogram traces. Based on the arrival times of the disturbances, we have inferred the overall propagation speed of the TIDs from Chelyabinsk to be 171 ± 14 m/s.

Keywords: Ionospheric disturbances, Wave propagation, Instruments and techniques, traveling ionospheric disturbances, gravity waves, ionosonde measurements

The Sun–Earth connection is studied using long-term measurements from the Sun and from the Earth. The auroral activity is shown to correlate to high accuracy with the smoothed sunspot numbers. Similarly, both geomagnetic activity and global surface temperature anomaly can be linked to cyclic changes in the solar activity. The interlinked variations in the solar magnetic activity and in the solar irradiance cause effects that can be observed both in the Earth's biosphere and in the electromagnetic environment. The long-term data sets suggest that the increase in geomagnetic activity and surface temperatures are related (at least partially) to longer-term solar variations, which probably include an increasing trend superposed with a cyclic behavior with a period of about 90 years.

The growing demand for fast access to accurate ionospheric electron density profiles and ionospheric characteristics calls for efficient dissemination of data from the many ionosondes operating around the globe. The global digisonde network with over 70 stations takes advantage of the Internet to make many of these sounders remotely accessible for data transfer and control. Key elements of the digisonde system data management are the visualization and editing tool SAO Explorer, the digital ionogram database DIDBase, holding raw and derived digisonde data under an industrial-strength database management system, and the automated data request execution system ADRES.

After a historical introduction in Section 1, the paper summarizes in Section 2 the physical principles that govern the behaviour of the ionospheric F2-layer. Section 3 reviews the physics of thermospheric dynamics at F-layer heights, and how the thermospheric winds affect the neutral chemical composition. Section 4 discusses the seasonal, annual and semiannual variations of the quiet F2 peak at midlatitudes, while Section 5 deals with storm conditions. The paper concludes by summing up the state of understanding of F2-layer variations and reviewing some important principles that apply to ionospheric studies

This work reviews some aspects of the ionospheric F-layer in the vicinity of the geomagnetic equator. Starting with a historical introduction, brief summaries are given of the physics that makes the equatorial ionosphere so interesting, concentrating on the large-scale structure rather than the smaller-scale instability phenomena. Several individual topics are then discussed, including eclipse effects, the asymmetries of the `equatorial trough', variations with longitude, the semiannual variation, the effects of the global thermospheric circulation, and finally the equatorial neutral thermosphere, including `superrotation' and possible topographic influences.

This paper briefly reviews questions relating to the equatorial thermosphere and ionosphere that do not seem to be fully solved. They include the effect of night E-region conductivity on F-region electrodynamics, annual and semiannual variations of F2-layer electron density, and superrotation' of the thermosphere. New results are presented on the neutral gas composition of the equatorial thermosphere, showing a pronounced annual variation.

The ionosphere displays variations on a wide range of time-scales, ranging from operational time-scales of hours and days up to solar cycles and longer. We use ionosonde data from thirteen stations to study the day-to-day variability of the peak F2-layer electron density, NmF2, which we use to define quantitative descriptions of variability versus local time, season and solar cycle. On average, for years of medium solar activity (solar decimetric flux approximately 140 units), the daily fluctuations of NmF2 have a standard deviation of 20% by day, and 33% by night. We examine and discuss the patterns of behaviour of ionospheric and geomagnetic variability, in particular the equinoctial peaks. For further analysis we concentrate on one typical midlatitude station, Slough. We find that the standard deviations of day-to-day and night-to-night values of Slough NmF2 at first increase with increasing length of the dataset, become fairly constant at lengths of 10-20 days and then increase further (especially at equinox) because of seasonal changes. We found some evidence of two-day waves, but they do not appear to be a major feature of Slough's F2 layer. Putting together the geomagnetic and ionospheric data, and taking account of the day-to-day variability of solar and geomagnetic parameters, we find that a large part of F2-layer variability is linked to that of geomagnetic activity, and attribute the rest to 'meteorological' sources at lower levels in the atmosphere. We suggest that the greater variability at night is due to enhanced auroral energy input, and to the lack of the strong photochemical control of the F2-layer that exists by day.

We compare the electron densities of two martian ionospheric layers, which we call M1 and M2, measured by Mars Global Surveyor during 9-27 March 1999, with the electron densities of the terrestrial E and F1 layers derived from ionosonde data at six sites. The day-to-day variations are all linked to changes in solar activity, and provide the opportunity of making the first simultaneous study of four photochemical layers in the solar system. The `ionospheric layer index', which we introduce to characterize ionospheric layers in general, varies between layers because different atmospheric chemistry and solar radiations are involved. The M2 and F1 layer peaks occur at similar atmospheric pressure levels, and the same applies to the M1 and E layers.

Adding together the northern and southern hemisphere values for pairs of stations, the combined peak electron density NmF2 is greater in December-January than in June-July. The same applies to the total height-integrated electron content. This "F2-layer annual asymmetry" between northern and southern solstices is typically 30%, and thus greatly exceeds the 7% asymmetry in ion production due to the annual variation of Sun-Earth distance. Though it was noticed in ionospheric data almost seventy years ago, the asymmetry is still unexplained. Using ionosonde data and also values derived from the International Reference Ionosphere, we show that the asymmetry exists at noon and at midnight, at all latitudes from equatorial to sub-auroral, and tends to be greater at solar minimum than solar maximum. We find a similar asymmetry in neutral composition in the MSIS model of the thermosphere. Numerical computations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP) model give a much smaller annual asymmetry in electron density and neutral composition than is observed. Including mesospheric tides in the model makes little difference. After considering possible explanations, which do not account for the asymmetry, we are left with the conclusion that dynamical influences of the lower atmosphere (below about 30 km), not included in our computations, are the most likely cause of the asymmetry.

The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external influences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric fields play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions affect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual effect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the effect of the composition changes is modified by the large solar zenith angle at midwinter. The zenith angle effect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle effect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/molecular ratio. This leads to a semiannual variation of NmF2. A different situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.

Ionosonde data from sixteen stations are used to study the semiannual and annual variations in the height of the ionospheric F2-peak, hmF2. The semiannual variation, which peaks shortly after equinox, has an amplitude of about 8 km at an average level of solar activity (10.7 cm flux = 140 units), both at noon and midnight. The annual variation has an amplitude of about 11 km at northern midlatitudes, peaking in early summer; and is larger at southern stations, where it peaks in late summer. Both annual and semiannual amplitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation of the peak electron density NmF2, and is not reproduced by the CTIP and TIME-GCM computational models of the quiet-day thermosphere and ionosphere. The semiannual variation in hmF2 is approximately "isobaric", in that its amplitude corresponds quite well to the semiannual variation in the height of fixed pressure-levels in the thermosphere, as represented by the MSIS empirical model. The annual variation is not "isobaric". The annual mean of hmF2 increases with solar 10.7 cm flux, both by night and by day, on average by about 0.45 km/flux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnetic activity, which affects thermospheric density and temperature and therefore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semiannual variation of tidal and wave energy transmitted to the thermosphere from lower levels in the atmosphere.

The solar flare of 23 February 1956 and the resulting geophysical disturbance ranks as one of the most remarkable solar-terrestrial events of the twentieth century. It sparked many papers and has seldom been equalled. Fifty years after the International Geophysical Year, it seems timely to review the observations of the event from today's perspective, and to draw on the recollections of scientists who were active at the time.

Keywords: 23 February 1956

A full understanding of how the heliospheric modulates the fluxes of galactic cosmic rays reaching the Earth is vital, not only for studies of their origin, acceleartion and propagation in our galaxy, but also for predicting their effects on modern technology and Earth's environment and organisms. We here use a strong 1.68-year oscillation in both GCR fluxes and the open solar magnetic flux to define where and how the heliospheric field shields Earth from GCRs and report an inward motion of that shield over the past 30 years.

An understanding of how the heliosphere modulates galactic cosmic ray (GCR) fluxes and spectra is important, not only for studies of their origin, acceleration and propagation in our galaxy, but also for predicting their effects (on technology and on the Earth's environment and organisms) and for interpreting abundances of cosmogenic isotopes in meteorites and terrestrial reservoirs. In contrast to the early interplanetary measurements, there is growing evidence for a dominant role in GCR shielding of the total open magnetic flux, which emerges from the solar atmosphere and enters the heliosphere. In this paper, we relate a strong 1.68-year oscillation in GCR fluxes to a corresponding oscillation in the open solar magnetic flux and infer cosmic-ray propagation paths confirming the predictions of theories in which drift is important in modulating the cosmic ray flux.

We use combinations of geomagnetic indices, based on both variation range and hourly means, to derive the solar wind flow speed, the interplanetary magnetic field strength at 1 AU and the total open solar flux between 1895 and the present. We analyze the effects of the regression procedure and geomagnetic indices used by adopting four analysis methods. These give a mean interplanetary magnetic field strength increase of 45.1 ±4.5% between 1903 and 1956, associated with a 14.4 ±0.7% rise in the solar wind speed. We use averaging timescales of 1 and 2 days to allow for the difference between the magnetic fluxes threading the coronal source surface and the heliocentric sphere at 1 AU. The largest uncertainties originate from the choice of regression procedure: the average of all eight estimates of the rise in open solar flux is 73.0 ±5.0%, but the best procedure, giving the narrowest and most symmetric distribution of fit residuals, yields 87.3 ±3.9%.

The ionosphere is a highly dynamic medium that exhibits weather disturbances at all latitudes, longitudes, and altitudes, and these disturbances can have detrimental effects on both military and civilian systems. In an effort to mitigate the adverse effects, we are developing a physics-based data assimilation model of the ionosphere and neutral atmosphere called the Global Assimilation of Ionospheric Measurements (GAIM). GAIM will use a physics-based ionosphere-plasmasphere model and a Kalman filter as a basis for assimilating a diverse set of real-time (or near real-time) measurements. Some of the data to be assimilated include in situ density measurements from satellites, ionosonde electron density profiles, occultation data, ground-based GPS total electron contents (TECs), two-dimensional ionospheric density distributions from tomography chains, and line-of-sight UV emissions from selected satellites. When completed, GAIM will provide specifications and forecasts on a spatial grid that can be global, regional, or local. The primary output of GAIM will be a continuous reconstruction of the three-dimensional electron density distribution from 90 km to geosynchronous altitude (35,000 km). GAIM also outputs auxiliary parameters, including NmF2, hmF2, NmE, hmE, and slant and vertical TEC. Furthermore, GAIM provides global distributions for the ionospheric drivers (neutral winds and densities, magnetospheric and equatorial electric fields, and electron precipitation patterns). In its specification mode, GAIM yields quantitative estimates for the accuracy of the reconstructed ionospheric densities.

A study of ionospheric data recorded at Slough/Chilton, UK, from 1935 to 2012, has revealed long-term changes in the relative strength of the annual and semi-annual variability in the ionospheric F2 layer critical frequencies. Comparing these results with data from the southern hemisphere station at Stanley in the Falkland Islands between 1945 and 2012 reveals a trend that appears to be anti-correlated with that at Chilton. The behaviour of foF2 is a function of thermospheric composition and so we argue that the observed long-term changes are driven by composition change. The ionospheric trends share some of their larger features with the trend in the variability of the aa geomagnetic index. Changes to the semi-annual/annual ratio in the Slough/Chilton and Stanley data may therefore be attributable to the variability in geomagnetic activity which controls the average latitudinal extent of the auroral ovals and subsequent thermospheric circulation patterns. Changes in ionospheric composition or thermospheric wind patterns are known to influence the height of the F2 layer at a given location. Long-term changes to the height of the F2 layer have been used to infer an ionospheric response to greenhouse warming. We suggest that our observations may influence such measurements and since the results appear to be dependent on geomagnetic longitude, this could explain why the long-term drifts observed in F2 layer height differ between locations.

Two decades ago, the U.S. Air Force Air Weather Service space forecasting group began generating what was termed an effective sunspot number (SSN_e) by fitting a model of the critical frequency of the F₂ layer (f_oF₂) to observed f_oF₂ values. Initially a preprocessing step in a larger analysis package, this parameter has taken on a life of its own and is now used in various applications for both forecasts and specification of the global f_oF₂ field. This paper describes the various ways in which this parameter is calculated, investigates the behavior of this parameter over solar cycle 21 (1976 through 1986), and compares it with other solar-ionospheric indices, including R₁₂, I_F2, I_G, and the Ionospheric Prediction Service (IPS) T index.

Observations of HF radar backscatter from artificial field-aligned irregularities in an ionosphere perturbed by travelling disturbances due to atmospheric gravity waves are presented. Some features of the spatio-temporal structure of the artificial radar backscatter can be explained in terms of the distortion of the ionosphere resulting from the travelling disturbances. The distorted ionosphere can allow the HF pump wave to access upper-hybrid resonance at larger distances from the transmitter than are normally observed and can also prevent the pump wave reaching this resonance at close distances. The variation in altitude of the irregularities sometimes results in a significant variation in the elevation angle of arrival of the backscattered signal at the radar implying that the radar "sees" a target moving in altitude. We suggest that this may be evidence of off-orthogonal scattering from the irregularities.

Noontime monthly median values of F2-layer critical frequency foF2 (m) for some ionospheric stations representing low- and mid-latitudes are examined for their dependence on solar activity for the years 1957 (IGY) to 1990. This is the period for which ionospheric data in digital form is available in two CD-ROMs at the World Data Center, Boulder. It is observed that at mid-latitudes, foF2 (m) shows nearly a linear relationship with R12 (the 12-month running average of the Zurich sunspot number), though this relation is nonlinear for low-latitudes. These results indicate some departures from the existing information often used in theoretical and applied areas of space research.

We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the aa index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earth's dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind-magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20–22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in aa observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the aa index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes.

A way of producing limited-area instantaneous maps of ionospheric characteristics is shown. An interpolation technique is applied for construction of the mapping model. The model combines monthly median maps of ionospheric characteristics and a set of measurements for a single moment of time that are exactly replicated during the mapping procedure. The accuracy of the mapping results is discussed, and samples of maps for different geophysical conditions for f_oF₂, f_oF₁, f_oE and M(3000)F₂ are presented.

The autocovariance prediction method has been used for ionospheric forecasting of f_oF₂ values for 1, 2, 4, 8, and 12 hours ahead at a single location. Time series of f_oF₂ data for ionospheric quiet and disturbed conditions for February 1986 and September and December 1990 at different European stations were studied in order to clarify the forecasting capabilities of the method for ionospheric purposes. The accuracy of the method varies within reasonable limits depending on the time range of the forecast for different conditions. Samples of the results for representative periods are presented. The forecast is compared with observations, monthly median recommendations of the Radiocommunication Sector of the International Telecommunication Union (ITU-R), and persistence models.

Abstract An assessment of the ionosphere perturbations can be made through the construction of the global instantaneous maps of the foF2 critical frequency (GIM-foF2) and the ionospheric weather index maps GIM-Wf. These maps can offer a potentially useful tool to provide users with a proper selection of the best radio wave propagation conditions over a certain area and also be used to help mitigate the effects of the disturbances on HF (High Frequency) communication and Global Navigation Satellite System positioning. This paper presents results of reconstruction of the ionospheric weather during five of the most intense superstorms observed since International Geophysical Year, IGY (1957, 1958, 1959, 1989, and 2003) with the instantaneous global maps of the F2 layer critical frequency, GIM-foF2, and the ionospheric weather index maps, GIM-Wf. The intensity of the ionospheric superstorm is characterized by the planetary Wfp index derived from GIM-Wf maps. Superposed epoch analysis of the extreme superstorms is made during 24 hr before the Wfp peak (time zero t₀ = 0 hr) and 48 hr afterwards. Model relationship is established between mean Wfp profile and geomagnetic superstorm profiles demonstrating saturation of the ionospheric storm activity toward the peak of geomagnetic storm. Time lag of Wfp_max is found equal to 9 hr after AE_max, 6 hr after ap_max and aa_max, and 2 hr after Dst_min, which allows model forecast of ionospheric superstorm when geomagnetic superstorm is captured with one or more of geomagnetic indices.

Keywords: Ionospheric Weather, Geomagnetic Superstorms, GIM-foF2, GIM-Wf

Extended cusp-like regions (ECRs) are surveyed, as observed by the Magnetospheric Ion Composition Sensor (MICS) of the Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) instrument aboard Polar between 1996 and 1999. The first of these ECR events was observed on 29 May 1996, an event widely discussed in the literature and initially thought to be caused by tail lobe reconnection due to the coinciding prolonged interval of strong northward IMF. ECRs are characterized here by intense fluxes of magnetosheath-like ions in the energy-per-charge range of ∼1 to 10 keV e^-1. We investigate the concurrence of ECRs with intervals of prolonged (lasting longer than 1 and 3 hours) orientations of the IMF vector and high solar wind dynamic pressure (P_SW). Also investigated is the opposite concurrence, i.e., of the IMF and high P_SW with ECRs. (Note that these surveys are asking distinctly different questions.) The former survey indicates that ECRs have no overall preference for any orientation of the IMF. However, the latter survey reveals that during northward IMF, particularly when accompanied by high P_SW, ECRs are more likely. We also test for orbital and seasonal effects revealing that Polar has to be in a particular region to observe ECRs and that they occur more frequently around late spring. These results indicate that ECRs have three distinct causes and so can relate to extended intervals in (1) the cusp on open field lines, (2) the magnetosheath, and (3) the magnetopause indentation at the cusp, with the latter allowing magnetosheath plasma to approach close to the Earth without entering the magnetosphere.

Observations of ten solar eclipses (1973 1999) enabled us to reveal and describe mutual relations between the white-light corona structures (e.g., global coronal forms and most conspicuous coronal features, such as helmet streamers and coronal holes) and the coronal magnetic field strength and topology. The magnetic field strength and topology were extrapolated from the photospheric data under the current-free assumption. In spite of this simplification the found correspondence between the white-light corona structure and magnetic field organization strongly suggests a governing role of the field in the appearance and evolution of local and global structures. Our analysis shows that the study of white-light corona structures over a long period of time can provide valuable information on the magnetic field cyclic variations. This is particularly important for the epoch when the corresponding measurements of the photospheric magnetic field are absent.

We report on the first realtime ionospheric predictions network and its capabilities to ingest a global database and forecast F-layer characteristics and “in situ” electron densities along the track of an orbiting spacecraft. A global network of ionosonde stations reported around-the-clock observations of F-region heights and densities, and an on-line library of models provided forecasting capabilities. Each model was tested against the incoming data; relative accuracies were intercompared to determine the best overall fit to the prevailing conditions; and the best-fit model was used to predict ionospheric conditions on an orbit-to-orbit basis for the 12-hour period following a twice-daily model test and validation procedure. It was found that the best-fit model often provided averaged (i.e., climatologically-based) accuracies better than 5% in predicting the heights and critical frequencies of the F-region peaks in the latitudinal domain of the TSS-1R flight path. There was a sharp contrast, however, in model-measurement comparisons involving predictions of actual, unaveraged, along-track densities at the 295 km orbital altitude of TSS-1R. In this case, extrema in the first-principle models varied by as much as an order of magnitude in density predictions, and the best-fit models were found to disagree with the “in situ” observations of Ne by as much as 140%. The discrepancies are interpreted as a manifestation of difficulties in accurately and self-consistently modeling the external controls of solar and magnetospheric inputs and the spatial and temporal variabilities in electric fields, thermospheric winds, plasmaspheric fluxes, and chemistry.

The global-scale modeling and measurement activities of the Sundial campaign of September 1986 are examined, and averaged, quiet-time, and dynamic ionospheric behaviors are investigated. Treatment is given to developments in empirical and first-principle models; and various aspects of magnetospheric-thermospheric-ionospheric coupling mechanisms are investigated. Overall results point to good empirical model specification of averaged F-region behavior, with suggestions for improvements in specification of layer peak densities near and across the sunset terminator. The difficulties in achieving a unique determination of electric fields, thermospheric winds, and plasmaspheric fluxes are elucidated in first-principle model attempts to reproduce global observations of quiet-time F-region heights and densities. In this connection, and in the treatment of magnetospherically-imposed electric field influences on low-latitude F-region dynamics, a greater need is shown for comprehensive measurements of auroral oval dynamics, thermospheric winds, electric fields, ion composition, and ionospheric layer heights and densities. The growing importance of the lower regions of the ionosphere and thermosphere and the associated controls of dynamo-driven electric fields are discussed.

Solar-terrestrial observations have been obtained in the SUNDIAL program during the October 5-13, 1984 period in order to explore cause and effect relationships controlling the global-scale ionosphere. It is suggested that the increased solar wind velocities noted are the result of a corotating high-speed stream coupled to a transequatorial solar coronal hole. The results are consistent with a step-wise coupling of processes from the coronal hole through the interplanetary and magnetospheric domains down to the equatorial ionosphere, where penetrating electric fields help trigger the most disturbed condition of equatorial spread-F.

We report on a study of three intense ionospheric storms that occurred in September 1989. Using Dst as a reference for storm onset and subsequent main and recovery phases, we analyze the observed worldwide responses of F region heights hmF2 and densities NmF2 as a function of universal and local times, latitudinal domains, and storm onset-times; and we compare the characteristics of all three storms. The following points are among the major findings: (1) The negative phase storm was the dominant characteristic, with the greatest intensity occurring in the regions which were in the nighttime hemisphere during the main phase; (2) at middle and low latitudes negative phase characteristics were observed first in the nighttime hemisphere and then corotated with the Earth into the dayside; (3) the most intense negative response occurred in the recovery phase; (4) observations of the negative phase characteristics supported thermospheric upwelling, increased mean molecular mass, and an associated enhancement in dissociative recombination as the principal cause-effect chain; but the observations suggest greater ion-neutral chemistry effects than accounted for in current models; (5) hmF2 was observed to respond quickly to the storm onset (pointing to the importance of electric fields) with enhanced values in all latitudinal and local time domains; (6) positive storm characteristics were among the issues most difficult to reconcile with current descriptions of cause-effect relationships; and (7) the analysis of all storm phases and comparisons with several modeling efforts show that future advances in understanding require a more accurate accounting of the influences of magnetospherically-imposed and dynamo-driven electric fields, plasmaspheric fluxes, and vibrationally excited N₂.

Covering the period from September 22 through October 4, 1986, the Sundial-86 Solar-Minimum Equinoctial Campaign studied the behavior of the global-scale ionosphere. The period covered the most quiet (Q1) and second most disturbed (D2) days of the entire month of September, with the disturbed conditions triggered by a high-speed solar wind stream. Ionospheric responses were monitored by the Sundial network of nearly 70 stations distributed approximately in three longitudinal domains; and global maps of f0F2 results were compared with the 'predictions' of the International Reference Ionosphere modified to include an empirical specification of auroral oval boundaries and associated high-latitude morphological domains. Comparisons that included regions in the polar cap, diffuse auroral oval, mid-latitude trough, equatorial anomaly, and the sunrise/sunset terminator showed good agreement between the hourly 8-day-averaged ionospheric observations and the model.

A superposed epoch analysis of geomagnetic storms has been undertaken. The storms are categorised via their intensity (as defined by the Dst index). Storms have also been classified here as either storm sudden commencements (SSCs) or storm gradual commencements (SGCs, that is all storms which did not begin with a sudden commencement). The prevailing solar wind conditions defined by the parameters solar wind speed (v_sw), density (ρ_sw) and pressure (P_sw) and the total field and the components of the interplanetary magnetic field (IMF) during the storms in each category have been investigated by a superposed epoch analysis. The southward component of the IMF, appears to be the controlling parameter for the generation of small SGCs (-100 nT< minimum Dst ≤ -50 nT for ≥ 4 h), but for SSCs of the same intensity solar wind pressure is dominant. However, for large SSCs (minimum Dst ≤ -100 nT for ≥ 4 h) the solar wind speed is the controlling parameter. It is also demonstrated that for larger storms magnetic activity is not solely driven by the accumulation of substorm activity, but substantial energy is directly input via the dayside. Furthermore, there is evidence that SSCs are caused by the passage of a coronal mass ejection, whereas SGCs result from the passage of a high speed/slow speed coronal stream interface. Storms are also grouped by the sign of B_z during the first hour epoch after the onset. The sign of B_z at t=+1 h is the dominant sign of the B_z for ∼24 h before the onset. The total energy released during storms for which B_z was initially positive is, however, of the same order as for storms where B_z was initially negative.

The increasing demand for upper-atmosphere nowcasting services for operational applications reveals the need for a realistic mapping of the ionosphere over Europe in real-time and especially during storm periods. To meet this need, a real-time updating method of simplified ionospheric regional model (SIRM) with autoscaled ionospheric characteristics observed by four European Digital Portable Sounders (DPS) ionosondes was recently developed. SIRM belongs to the group of ionospheric models for the standard vertical incidence (VI) ionospheric characteristics such as the critical frequency of the ionospheric F2 layer foF2 and the propagation factor M(3000)F2, which oversimplify a number of the ionospheric phenomena of real significance for radio communications applications showing satisfactory performance for median ionospheric condition description in restricted area of mid-latitudes. As a step forward, the rapid conversion of real-time data from four European digisondes to the driving parameters of the SIRM was introduced as the real-time SIRM updating (SIRMUP). In SIRMUP approach, the values of the ionospheric characteristics from first-guess model parameters at measurement points are combined with real-time measurements. The reliability of the real-time SIRM update method has already been tested in terms of the foF2 for various ionospheric conditions and the simulation results were very promising. In this paper, the simulation tests are continued in order to investigate the efficiency of the SIRMUP method in mapping the propagation conditions over Europe as they are expressed by the propagation factor M(3000)F2. In general, the results demonstrate that SIRMUP procedure has the potential to be used in real time for nowcasting the standard ionospheric characteristics over Europe, for operational applications.

Keywords: Ionospheric radio-propagation; Ionospheric mapping; Ionospheric modelling; Mid-latitude ionosphere

The possible effects of the orientation of the IMF on the ionosphere has been studied by Tulunay (1995) using foF2 data from 15 ionospheric stations in Europe over the COST 238 area. The results showed that a good amount of the day to day variability of the mid-latitude ionospheric F region could be related to changes in orientation of the southward IMF Bz. This variability is quantified as the maximum change of deltafoF2. This paper investigates the effects of By distribution on the ionospheric critical frequencies.

Potential effects of the IMF-orientation on the mid-latitude ionosphere are further investigated using critical frequencies foF2 from six ionosonde stations. For a period of 15 days around each inversion of BZ, excluding all days with Ap >=6, a quiet standard diurnal variation was determined by day-by-day averaging for each hour UT. The regular diurnal, seasonal and solar cycle variations were then removed from the data by substracting from these the quiet standard value. The so obtained differences foF2 were sorted after the IMF polarity. Distinct effects of northward and southward inversions were found so that a large part of the day-to day variability may be attributed to IMF BZ polarity changes.

Using critical frequencies, f0F2 from the Lannion, Slough, Poitiers, Garchy, Dourbes, Rome, Juliusrud, Gibilmanna, Pruhonice, Uppsala, Kaliningrad, Miedzeszyn, Sofia, Athens and Kiev ionosonde stations, the possible effects of the orientation of the Interplanetary Magnetic Field (IMF) on mid-latitude ionosphere are further investigated. This time, only the southward polarity changes in IMF Bz with seasonal effects were considered. The same method of analysis was employed to facilitate a comparison between the recent results presented here with those which appeared in the preceding papers in the series. That is, the regular diurnal, seasonal and solar cycle variations in the f0F2 data were removed by subtracting the mean of the f0F2 for the same UT on all magnetically quite days (Ap < 6) within 15 days around the IMF Bz turnings (Tulunay, 1994). This last paper also includes the seasonal effects on the ionospheric data. The results confirm that much of the day-to-day variability of the mid-latitude ionosphere may be related to the orientation of the southward IMF Bz , characterized by the ionospheric winter anomaly. Day-to-day ionospheric variability becomes more significant towards higher latitudes.

Although sunspot-number series have existed since the mid-nineteenth century, they are still the subject of intense debate, with the largest uncertainty being related to the “calibration” of the visual acuity of individual observers in the past. A daisy-chain regression method is usually applied to inter-calibrate the observers, which may lead to significant bias and error accumulation. Here we present a novel method for calibrating the visual acuity of the key observers to the reference data set of Royal Greenwich Observatory sunspot groups for the period 1900–1976, using the statistics of the active-day fraction. For each observer we independently evaluate their observational thresholds [ S S $S_{\mathrm{S}}$ ] defined such that the observer is assumed to miss all of the groups with an area smaller than S S $S_{\mathrm{S}}$ and report all the groups larger than S S $S_{\mathrm{S}}$ . Next, using a Monte-Carlo method, we construct a correction matrix for each observer from the reference data set. The correction matrices are significantly non-linear and cannot be approximated by a linear regression or proportionality. We emphasize that corrections based on a linear proportionality between annually averaged data lead to serious biases and distortions of the data. The correction matrices are applied to the original sunspot-group records reported by the observers for each day, and finally the composite corrected series is produced for the period since 1748. The corrected series is provided as supplementary material in electronic form and displays secular minima around 1800 (Dalton Minimum) and 1900 (Gleissberg Minimum), as well as the Modern Grand Maximum of activity in the second half of the twentieth century. The uniqueness of the grand maximum is confirmed for the last 250 years. We show that the adoption of a linear relationship between the data of Wolf and Wolfer results in grossly inflated group numbers in the eighteenth and nineteenth centuries in some reconstructions.

We examined geomagnetic field observations at low and middle latitudes in the Northern Hemisphere during a 50-min interval (12 May 1999), characterized by a complex behaviour of the solar wind dynamic pressure. For the entire interval, the aspects of the geomagnetic response can be organized into four groups of events which show common characteristics for the H and D components, respectively. The correspondence between the magnetospheric field and the ground components reveals different aspects of the geomagnetic response in different magnetic local time (MLT) sectors. For the H component, the correspondence is highly significant in the dusk and night sectors; in the dawn and prenoon sectors it shows a dramatic change across a separation line that extends approximately between (6 MLT, 35^o) and (13 MLT, 60^o). For the D component, the correspondence has significant values in the dawn and prenoon regions. We propose a new approach to the experimental data analysis which reveals that, at each station, the magnetospheric field has a close correspondence with the geomagnetic field projection along an axis (M1) that progressively rotates from north/south (night events) to east/west orientation (dawn events). When projected along M1, the geomagnetic signals can be interpreted in terms of a one-dimensional pattern that mostly reflects the field behaviour observed at geostationary orbit. Several features appear more evident in this perspective, and the global geomagnetic response to the SW pressure variations appears much clearer than in other representations. In particular, the MLT dependence of the geomagnetic response is much smaller than that one estimated by previous investigations. A clear latitudinal dependence emerges in the dusk sector. The occurrence of low frequency waves at ∼2.8mHz can be interpreted in terms of global magnetospheric modes driven by the SW pulse. This event occurred in the recovery phase after the day the SW almost disappeared (11 May 1999): in this sense our results suggest a rapid recovery of almost typical magnetospheric conditions soon after a huge expansion. Overshoot amplitudes, greater than in other cases, are consistent with a significant reduction of the ring current.

n this paper we investigate the relationship between polar cap sporadic-E layers and the direction of the interplanetary magnetic field (IMF) using a 2-year database from Longyearbyen (75.2 CGM Lat, Svalbard) and Thule (85.4 CGM Lat, Greenland). It is found that the MLT distributions of sporadic-E occurrence are different at the two stations, but both are related to the IMF orientation. This relationship, however, changes from the centre of the polar cap to its border. Layers are more frequent during positive By at both stations. This effect is particularly strong in the central polar cap at Thule, where a weak effect associated with Bz is also observed, with positive Bz correlating with a higher occurrence of Es. Close to the polar cap boundary, at Longyearbyen, the By effect is weaker than at Thule. On the other hand, Bz plays there an equally important role as By, with negative Bz correlating with the Es occurrence. Since Es layers can be created by electric fields at high latitudes, a possible explanation for the observations is that the layers are produced by the polar cap electric field controlled by the IMF. Using electric field estimates calculated by means of the statistical APL convection model from IMF observations, we find that the diurnal distributions of sporadic-E occurrence can generally be explained in terms of the electric field mechanism. However, other factors must be considered to explain why more layers occur during positive than during negative By and why the Bz dependence of layer occurrence in the central polar cap is different from that at the polar cap boundary.

Current practical literature for HF NVIS communication places significant emphasis on foF2 as being the maximum frequency for vertical propagation. This, however, fails to consider the extraordinary wave. This paper presents the analysis of 5 MHz beacon data, showing the relevance of the extraordinary wave in the MUF calculation for NVIS propagation. The results are in full agreement with established scientific theory and ionospheric propagation prediction methods, the detail of which the HF NVIS user community may not be aware of.

Keywords: HF NVIS communication;frequency 5 MHz;ionospheric propagation prediction method;near-vertical incidence skywave propagation;extraordinary wave mode;vertical propagation;

The interaction between the Earth's magnetic field and the solar wind plasma results in a natural plasma confinement system which stores energy. Dissipation of this energy through Joule heating in the ionosphere can be studied via the Auroral Electrojet (AE) index. The apparent broken power law form of the frequency spectrum of this index has motivated investigation of whether it can be described as fractal coloured noise. One frequently-applied test for self-affinity is to demonstrate linear scaling of the logarithm of the structure function of a time series with the logarithm of the dilation factor λ. We point out that, while this is conclusive when applied to signals that are self-affine over many decades in λ, such as Brownian motion, the slope deviates from exact linearity and the conclusions become ambiguous when the test is used over shorter ranges of λ. We demonstrate that non self-affine time series made up of random pulses can show near-linear scaling over a finite dynamic range such that they could be misinterpreted as being self-affine. In particular we show that pulses with functional forms such as those identified by Weimer within the AL index, from which AE is partly derived, will exhibit nearly linear scaling over ranges similar to those previously shown for AE and AL. The value of the slope, related to the Hurst exponent for a self-affine fractal, seems to be a more robust discriminator for fractality, if other information is available.

Attention is drawn to the existence of errors in the original digital dataset containing sunspot data extracted from certain sections of the printed Greenwich Photo-heliographic Results (GPR) 1874–1976. Calculating the polar coordinates from the heliographic coordinates and comparing them with the recorded polar coordinates reveals that there are both isolated and systematic errors in the original sunspot digital dataset, particularly during the early years (1874–1914). It should be noted that most of these errors are present in the compiled sunspot digital dataset and not in the original printed copies of the Greenwich Photo-heliographic Results. Surprisingly, many of the errors in the digitised positions of sunspot groups are apparently in the measured polar coordinates, not the derived heliographic coordinates. The mathematical equations that are used to convert between heliographic and polar coordinate systems are formulated and then used to calculate revised (digitised) polar coordinates for sunspot groups, on the assumption that the heliographic coordinates of every sunspot group are correct. The additional complication of requiring accurate solar ephemerides in order to solve the mathematical equations is discussed in detail. It is shown that the isolated and systematic errors, which are prevalent in the sunspot digital dataset during the early years, disappear if revised polar coordinates are used instead. A comprehensive procedure for checking the original sunspot digital dataset is formulated in an Appendix.

Potential sources of inhomogeneity in the sunspot measurements published by the Royal Observatory, Greenwich, during the early interval 1874–1885 are examined critically. Particular attention is paid to inhomogeneities that might arise because the sunspot measurements were derived from solar photographs taken at various contributing solar observatories, which used different telescopes, experienced different seeing conditions, and employed different photographic processes. The procedures employed in the Solar Department at the Royal Greenwich Observatory (RGO), Herstmonceux, during the final phase of sunspot observations provide a modern benchmark for interpreting the early sunspot measurements. The different observing telescopes used at the contributing solar observatories during the interval 1874–1885 are discussed in detail, using information gleaned from the official RGO publications and other relevant historical documents. Likewise, the different photographic processes employed at the different solar observatories are reviewed carefully. The procedures used by RGO staff to measure the positions and areas of sunspot groups on photographs of the Sun having a nominal radius of either four or eight inches are described. It is argued that the learning curve for the use of the Kew photoheliograph at the Royal Observatory, Greenwich, actually commenced in 1858, not 1874. The RGO daily number of sunspot groups is plotted graphically and analysed statistically. Similarly, the changes of metadata at each solar observatory are shown on the graphical plots and analysed statistically. It is concluded that neither the interleaving of data from the different solar observatories nor the changes in metadata invalidates the RGO count of the number of sunspot groups, which behaves as a quasi-homogeneous time series. Furthermore, it is emphasised that the correct treatment of days without photographs is quite crucial to the correct calculation of Group Sunspot Numbers.

The daily number of sunspot groups on the solar disk, as recorded by the programme of sunspot observations performed under the aegis of the Royal Observatory, Greenwich, UK, and subsequently the Royal Greenwich Observatory (RGO), is re-examined for the interval 1874–1885. The motivation for this re-examination is the key role that the RGO number of sunspot groups plays in the calculation of Group Sunspot Numbers (Hoyt and Schatten in Solar Phys. 179, 189, 1998a; Solar Phys. 181, 491, 1998b). A new dataset has been derived for the RGO daily number of sunspot groups in the interval 1874–1885. This new dataset attempts to achieve complete consistency between the sunspot data presented in the three main sections of the RGO publications and also incorporates all known errata and additions. It is argued that days for which no RGO solar photograph was acquired originally should be regarded, without exception, as being days without meaningful sunspot data. The daily number of sunspot groups that Hoyt and Schatten assign to days without RGO photographs is frequently just a lower limit. Moreover, in the absence of a solar photograph, the daily number of sunspot groups is inevitably uncertain because of the known frequent occurrence of sunspot groups that exist for just a single day. The elimination of days without photographs changes the list of inter-comparison days on which both the primary RGO observer and a specified secondary comparison observer saw at least one sunspot group. The resulting changes in the personal correction factors of secondary observers then change the personal correction factors of overlapping tertiary observers, etc. In this way, numerical changes in the personal correction factors of secondary observers propagate away from the interval 1874–1885, thereby potentially changing the arithmetical calculation of Group Sunspot Numbers over an appreciably wider time interval.

The measurements of sunspot positions and areas that were published initially by the Royal Observatory, Greenwich, and subsequently by the Royal Greenwich Observatory (RGO), as the Greenwich Photo-heliographic Results (GPR), 1874–1976, exist in both printed and digital forms. These printed and digital sunspot datasets have been archived in various libraries and data centres. Unfortunately, however, typographic, systematic and isolated errors can be found in the various datasets. The purpose of the present paper is to begin the task of identifying and correcting these errors. In particular, the intention is to provide in one foundational paper all the necessary background information on the original solar observations, their various applications in scientific research, the format of the different digital datasets, the necessary definitions of the quantities measured, and the initial identification of errors in both the printed publications and the digital datasets. Two companion papers address the question of specific identifiable errors; namely, typographic errors in the printed publications, and both isolated and systematic errors in the digital datasets. The existence of two independently prepared digital datasets, which both contain information on sunspot positions and areas, makes it possible to outline a preliminary strategy for the development of an even more accurate digital dataset. Further work is in progress to generate an extremely reliable sunspot digital dataset, based on the programme of solar observations supported for more than a century by the Royal Observatory, Greenwich, and the Royal Greenwich Observatory. This improved dataset should be of value in many future scientific investigations.

The validity of a technique developed by the authors to identify historical occurrences of intense geomagnetic storms, which is based on finding approximately coincident observations of sunspots and aurorae recorded in East Asian histories, is corroborated using more modern sunspot and auroral observations. Scientific observations of aurorae in Japan during the interval 1957–2004 are used to identify geomagnetic storms that are sufficiently intense to produce auroral displays at low geomagnetic latitudes. By examining white-light images of the Sun obtained by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the Debrecen Heliophysical Observatory and the Solar and Heliospheric Observatory spacecraft, it is found that a sunspot large enough to be seen with the unaided eye by an "experienced" observer was located reasonably close to the central solar meridian immediately before all but one of the 30 distinct Japanese auroral events, which represents a 97% success rate. Even an "average" observer would probably have been able to see a sunspot with the unaided eye before 24 of these 30 events, which represents an 80% success rate. This corroboration of the validity of the technique used to identify historical occurences of intense geomagnetic storms is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual historical geomagnetic storms during the greater part of the past two millennia.

A further study is made of the validity of a technique developed by the authors to identify historical occurrences of intense geomagnetic storms, which is based on finding approximately coincident observations of sunspots and aurorae recorded in East Asian histories. Previously, the validity of this technique was corroborated using scientific observations of aurorae in Japan during the interval 1957–2004 and contemporaneous white-light images of the Sun obtained by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the Debrecen Heliophysical Observatory, and the Solar and Heliospheric Observatory spacecraft. The present investigation utilises a list of major geomagnetic storms in the interval 1868–2008, which is based on the magnitude of the AA* magnetic index, and reconstructed solar images based on the sunspot observations acquired by the Royal Greenwich Observatory during the shorter interval 1874–1976. It is found that a sunspot large enough to be seen with the unaided eye by an "experienced" observer was located reasonably close to the central solar meridian for almost 90% of these major geomagnetic storms. Even an "average" observer would easily achieve a corresponding success rate of 70% and this success rate increases to about 80% if a minority of ambiguous situations are interpreted favourably. The use of information on major geomagnetic storms, rather than modern auroral observations from Japan, provides a less direct corroboration of the technique for identifying historical occurrences of intense geomagnetic storms, if only because major geomagnetic storms do not necessarily produce auroral displays over East Asia. Nevertheless, the present study provides further corroboration of the validity of the original technique for identifying intense geomagnetic storms. This additional corroboration of the original technique is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual geomagnetic storms during the greater part of the past two millennia.

All the accessible auroral observations recorded in Chinese and Japanese histories during the interval AD 1840–1911 are investigated in detail. Most of these auroral records have never been translated into a Western language before. The East Asian auroral reports provide information on the date and approximate location of each auroral observation, together with limited scientific information on the characteristics of the auroral luminosity such as colour, duration, extent, position in the sky and approximate time of occurrence. The full translations of the original Chinese and Japanese auroral records are presented in an appendix, which contains bibliographic details of the various historical sources. (There are no known reliable Korean observations during this interval.) A second appendix discusses a few implausible "auroral" records, which have been rejected. The salient scientific properties of all exactly dated and reliable East Asian auroral observations in the interval AD 1840–1911 are summarised succinctly. By comparing the relevant scientific information on exactly dated auroral observations with the lists of great geomagnetic storms compiled by the Royal Greenwich Observatory, and also the tabulated values of the Ak (Helsinki) and aa (Greenwich and Melbourne) magnetic indices, it is found that 5 of the great geomagnetic storms (aa>150 or Ak>50) during either the second half of the nineteenth century or the first decade of the twentieth century are clearly identified by extensive auroral displays observed in China or Japan. Indeed, two of these great storms produced auroral displays observed in both countries on the same night. Conversely, at least 29 (69%) of the 42 Chinese and Japanese auroral observations occurred at times of weak-to-moderate geomagnetic activity (aa or Ak≤50). It is shown that these latter auroral displays are very similar to the more numerous (about 50) examples of sporadic aurorae observed in the United States during the interval AD 1880–1940. The localised nature and spatial structure of some sporadic aurorae observed in East Asia is indicated by the use of descriptive terms such as "lightning", "rainbow", "streak" and "grid".

The paper uses the statistics of extreme values to investigate the statistical properties of the largest areas of sunspots and photospheric faculae per solar cycle. The largest values of the synodic-solar-rotation mean areas of umbrae, whole spots and faculae, which have been recorded for nine solar cycles are shown to comply with the general form of the extreme value probability function. Empirical expressions are derived for the three extreme value populations from which the characteristic statistic parameters, namely the mode, median, mean and standard deviation, can be calculated for each population. It is found that extreme areas of umbrae and whole spots have a diversion comparable to that found by Siscoe for the extreme values of sunspot number whereas the extreme areas of faculae have a smaller dispersion which is comparable to that found by Siscoe for the largest geomagnetic storm per solar cycle.

The use of time delay feed-forward neural networks to predict the hourly values of the ionospheric F-2 layer critical frequency, f_oF₂, 24 hours ahead, have been examined. The 24 measurements of f_oF₂ per day are reduced to five coefficients with principal component analysis. A time delay line of these coefficients is then used as input to a feed-forward neural network. Also included in the input are the 10.7 cm solar flux and the geomagnetic index Ap. The network is trained to predict measured f_oF₂ data from 1965 to 1985 at Slough ionospheric station and validated on an independent validation set from the same station for the periods 1987-1990 and 1992-1994. The results are compared with two different autocorrelation methods for the years 1986 and 1991, which correspond to low and high solar activity, respectively.

Keywords: SOLAR-WIND DATA; GEOMAGNETIC STORMS

A time-weighted accumulation of the ap index, ap(τ) (Wrenn, 1987; Wrenn et al., 1987, 1989), together with other similar indices, was explored as a predictor of ionospheric behaviour, using f_oF₂ data for a selection of locations in Australia and Europe for September and October 1989. All the time accumulated indices showed improved linear correlations, indicative of a response time of the order of about 15 hours. The response time could be decomposed into a lag between respective time series and a persistence time, although the decomposition appeared unnecessary as the persistence time carried the same information. Of the individual indices investigated, aa(τ) appeared best and the auroral oval equatorward edge index (AI index) was poorest, although the differences were not statistically significant. Comparisons between the aa, ap and Kp indices, plus comparisons between different ionospheric parameters showed that forecasting may be improved using different transformations of the data. While these results appear good, further studies using other stations and seasons are warranted to confirm their utility for forecasting.

The annual and semi-annual variations of the ionosphere are investigated in the present paper by using the daytime F2 layer peak electron concentration (NmF2) observed at a global ionosonde network with 104 stations. The main features are outlined as follows. (1) The annual variations are most pronounced at magnetic latitudes of 40-60^o in both hemispheres, and usually manifest as winter anomalies; Below magnetic latitude of 40^o as well as in the tropical region they are much weaker and winter anomalies that are not obvious. (2) The semi-annual variations, which are usually peak in March or April in most regions, are generally weak in the near-pole regions and strong in the far-pole regions of both hemispheres. (3) Compared with their annual components, the semi-annual variations in the tropical region are more significant. In order to explain the above results, we particularly analyze the global atomic/molecular ratio of [O/N₂] at the F2 layer peak height by the MSIS90 model. The results show that the annual variation of [O/N₂] is closely related with that of NmF2 prevailing in mid-latitudes and [O/N₂] annual variation usually may lead to the winter anomalies of NmF2 occurring in the near-pole region. Moreover, NmF2 semi-annual variations appearing in the tropical region also have a close relationship with the variation of [O/N₂]. On the other hand, the semi-annual variations of NmF2 in the far-pole region cannot be simply explained by that of [O/N₂], but the variation of the solar zenith angle may also have a significant contribution.

A method for mapping of ionospheric conditions over Europe, suitable to be used in real time for operational applications, is described in this paper. The method is based on the Simplified Ionospheric Regional Model ( SIRM), a regional model of the standard vertical incidence monthly median ionospheric characteristics that has been updated with real-time ( automatic scaled) ionospheric observations to produce nowcasting maps over Europe. As substantial fluctuations from a monthly median regional ionospheric description occur on day-to-day basis, the SIRM results oversimplified a number of the ionospheric phenomena of real significance for radio communications applications. Therefore a rapid conversion of real-time data from four European digisondes ( Digital Portable Sounders) to the driving parameters of the Simplified Ionospheric Regional Model is introduced as the real-time SIRM updating (SIRMUP). In this approach, values of the ionospheric characteristics from first-guess model parameters at measurement points are combined with real-time measurements. To assess the qualitative improvements achieved with the real-time SIRM update method, observations of foF2 parameter with SIRMUP predictions were compared for various ionospheric conditions. The simulation shows that the SIRMUP prediction results are much improved comparing to SIRM predictions, especially during large-scale ionospheric disturbances, as well as during quiet conditions, while there was a marginal improvement during localized ionospheric disturbances. In general, the results clearly demonstrate that the proposed procedure of updating SIRM with automatic scaling ionospheric parameters from the four European digisondes has the potential to be used in real time for nowcasting the standard ionospheric characteristics over Europe for operational applications.

Annual, seasonal and semiannual variations of F2-layer electron density (NmF2) and height (hmF2) have been compared with the coupled thermosphere-ionosphere-plasmasphere computational model (CTIP), for geomagnetically quiet conditions. Compared with results from ionosonde data from midlatitudes, CTIP reproduces quite well many observed features of NmF2, such as the dominant winter maxima at high midlatitudes in longitude sectors near the magnetic poles, the equinox maxima in sectors remote from the magnetic poles and at lower latitudes generally, and the form of the month-to-month variations at latitudes between about 60^oN and 50^oS. CTIP also reproduces the seasonal behaviour of NmF2 at midnight and the summer-winter changes of hmF2. Some features of the F2-layer, not reproduced by the present version of CTIP, are attributed to processes not included in the modelling. Examples are the increased prevalence of the winter maxima of noon NmF2 at higher solar activity, which may be a consequence of the increase of F2-layer loss rate in summer by vibrationally excited molecular nitrogen, and the semiannual variation in hmF2, which may be due to tidal effects. An unexpected feature of the computed distributions of NmF2 is an east-west hemisphere