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 80degrees, they are coherent some 6000 km apart, and they occur about 12 respectively. The typical longitudinal size of PWS with periods of about 10 and 13 days is 100degrees, they are coherent some 7500 km apart, and they occur about 24 22 with periods of about 16 days seem to be global scale phenomena, and they occur about 30 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 2-3, 5-6, 10 and 16 days, but even up to 65-70 with periods of about 10 and 16 days, and they practically drive 100 planetary wave activity in the mesosphere/lower thermosphere (MLT) winds can drive about 20-30 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.
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.
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_oF_2) 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] 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 (foF2) 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 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.
[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 ap and is designed to scale the normal quiet-time F layer critical frequency (f(o)F(2)) 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 ap > 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 the storm days and is able to capture more than 50 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(o)F(2)) 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 (foF2) 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.
An ionospheric F-2 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 degrees 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 Kp <= 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 (Kp <= 2(+)) and disturbed (Kp > 3(-)). For all latitudes and levels of geomagnetic activity, the lowest variability was typically found in summer (10-15 (15-40 extremes. The exception was low latitudes at equinox, which had surprising low variability (10 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.
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. (c) 2005 Elsevier Ltd. All rights reserved.
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 N-2 components decrease by 20 and 40 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 (51.3degreesN, 1degreesW) during the eclipse of August 11, 1999. The decrease in foF2 reaches similar to1 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 N-2 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.
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.
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.
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 E 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 E) 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.
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.
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 degreesN, 14.6 degreesE), 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.
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.
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 of latitude and 30 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.
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 to0.1 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.
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.
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∘06'N, 5∘18'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 252 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 Srmjle (63∘68'N, 20∘20'E) and yielded a corresponding minimum of 162 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.
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 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).
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 to 1-2 days) and approx. +/-15-20 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 (K-p > 4), the average 1-sigma variability of N-max about the mean ranges from approx. +/-35 (anomaly peak) to approx. +/-55 frequencies, and from approx. +/-25 (high-latitudes) at low frequencies. Some estimates are also provided on N-max variability connected with annual, semiannual and Ii-year solar cycle variations. (C) 2000 Elsevier Science Ltd. All rights reserved.
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 (in terms of overall model accuracy rather than relative to each other), 3.8 interpolation, and 34.3 interpolation. Utilizing the interpolation algorithm lowers the RMS error by 26 complete data, are used as an input to both the interpolated and the uninterpolated models.
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.
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.
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. (c) 2005 Elsevier Ltd. All rights reserved.
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.
Based on a 37-year-long (1958-1994) series of hourly measurements of the ionospheric E-region critical frequency f(0)E 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.
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(c)F2 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.
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.
An empirical model is developed to describe the variations of midlatitude F region ionization along all longitudes within the dip latitude band (30 induced by geomagnetic activity, by using the relative deviations () of the F region critical frequency f0F2 from its monthly median. The geomagnetic activity is represented by the Kp index. The main statistical relationship between and Kp is obtained by using 11 years of data from 26 midlatitude ionosondes. The statistical analysis reveals that the average dependence of on Kp 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 along local times can be assumed sinusoidal. A continuity equation is written for with the production term being a function of Kp modulated by a sinusoidal function of local time and the loss term proportional to with a loss coefficient =1/T. A new, modified function of geomagnetic activity (Kf) is introduced, being proportional to 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 Kf. 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%.
[1] The authors expand the previously developed midlatitude model, providing the relative deviation of f(o)F(2) 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.
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/N-2] and confirmed that the noon foF2 annual variations prevailing in mid-high latitudes are caused largely by the annual variation of [O/N-2]. As the noon foF2 semi-annual variations pronounced in far pole regions, we should consider the contribution of [O/N-2], 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/N-2], 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 -10degrees and +10degrees of the geomagnetic latitude, Phi), in latitude range 2 (10degrees < &UPhi; &LE; 30&DEG;), in latitude range 3 (30&DEG; < φ less than or equal to 45degrees, 30degrees < &UPhi; &LE; 45&DEG;), in latitude range 4 (45&DEG; < phi less than or equal to 60degrees, 45degrees < &UPhi; &LE; 60&DEG;), and in latitude range 5 (60&DEG; < Φ less than or equal to 90degrees), where phi 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 I 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.
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 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 10degreesW-15degreesE, 55degreesE-85degreesE, 135degreesE-155degreesE and 200degreesE-255degreesE 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
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/N-2] 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.
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).
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.
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 Kr = - 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 (RM) is assumed to contain a component (Rv) 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 (Rx) may be regarded as an error which has a Standard Deviation of about 20 per cent. A new index (IF2) has been constructed for the period 1938 also be regarded as giving an approximate value of Rv, but its residual error component (Rz) has an S.D. which is only about one tenth that of Rx. The magnitude of IF2, 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 Rv, 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.
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.
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. (c) 2005 Elsevier Ltd. All rights reserved.
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 35degreesN - 70degreesN and 5degreesW - 40degreesE. 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.
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.
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 day, and 33 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.
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 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 N m F 2, h m F 2, 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.
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(0)F(2), f(0)F(1), f(0)E and M(3000)F-2 are presented.
The autocovariance prediction method has been used for ionospheric forecasting of f(o)F(2) values for 1, 2, 4, 8, and 12 hours ahead at a single location. Time series of f(o)F(2) 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.
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 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 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.
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.
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.
The use of time delay feed-forward neural networks to predict the hourly values of the ionospheric F-2 layer critical frequency, f(0)F(2), 24 hours ahead, have been examined. The 24 measurements of f(0)F(2) 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(0)F(2) 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() (Wrenn, 1987; Wrenn et al., 1987, 1989), together with other similar indices, was explored as a predictor of ionospheric behaviour, using 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() 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.
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 60N and 50S. 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
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