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 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
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.
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.
New ionospheric activity indices are derived from automatically scaled online data from several European ionosonde stations. These indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). The most reliable indices are derived from the maximum electron density of the ionospheric F 2-layer expressed by the maximum critical frequency foF 2. Similar indices derived from ionospheric M(3000)F 2 values show a markedly lower variability indicating that the changes of the altitude of the F 2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the F 2-layer. By using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of Europe (34°N¿60°N; 5°W¿40°E) can now be displayed online.
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.
A prominent large-scale ionospheric disturbance was observed in the European mid-latitude sector during the recent extreme space weather event in October 2003. Measurements of the horizontal component of the geomagnetic field H, the critical frequency of the F2 layer foF2, and the vertical total electron content (TEC) from the European network of observational sites are used to describe the temporal and spatial storm evolution process. It is found that the ionospheric F region storm morphology was dominated by negative disturbances over high mid-latitudes and positive disturbance at low mid-latitudes during the initial phase and by overall negative disturbances during the main phase. Although a good agreement between the two independent measurements was detected by comparing the storm-time behaviour of foF2 and TEC during the main phase of the storm, some irregularities have been recognised in TEC variations at high mid-latitudes. The relative merit of real-time observational solar-terrestrial data for accurate specification of the geomagnetically disturbed ionospheric F region during the extreme space weather conditions is discussed.
The robustness of the aa geomagnetic index is of critical importance to the debate about the previously reported doubling of the solar coronal magnetic field in the last 100 years, inferred from an increasing trend in this index. To test the trend in aa, we have reconstructed the aa index using two long-running European stations (Sodankyla from 1914 and Niemegk from 1890) to provide data for the northern component of the index that are independent of data from the UK observatories used in the official'' aa index. Both the fully reconstructed'' aa series, based on Sodankyla ( 67 degrees N, L = 5.2 R-E) and Niemegk ( 52 degrees N, L = 2.3 R-E) data in combination with the official aa Southern Hemisphere data, confirm the increasing trend in the index. The Niemegk-based index shows little solar cycle variation in its deviation from the official index, probably because of the midlatitude location of the station. The high-latitude station, Sodankyla, is more affected by active geomagnetic conditions during solar maximum because of the proximity of the auroral oval to the station. Nevertheless, its index also clearly confirms the increasing trend in the aa index and hence supports the idea of a long-term increase in solar coronal magnetic field strength. As an added test, we reconstructed the aa index from a single site using data from two long-running UK stations, Eskdalemuir and Lerwick, applying a technique known as interhourly variation (IHV) proposed by Svalgaard et al. ( 2004). The resulting series is designed to be primarily sensitive to solar wind conditions. Both the reconstructed aa(IHV) also showed an increasing trend with time and high consistency with the official aa index. Overall, we conclude that the robustness of the trend in the aa index supports the idea of a long-term increase in solar coronal magnetic field strength.
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.
There are no direct observational methods for determining the total rate at which energy is extracted from the solar wind by the magnetosphere. In the absence of such a direct measurement, alternative means of estimating the energy available to drive the magnetospheric system have been developed using different ionospheric and magnetospheric indices as proxies for energy consumption and dissipation and thus the input. The so-called coupling functions are constructed from the parameters of the interplanetary medium, as either theoretical or empirical estimates of energy transfer, and the effectiveness of these coupling functions has been evaluated in terms of their correlation with the chosen index. A number of coupling functions have been studied in the past with various criteria governing event selection and timescale. The present paper contains an exhaustive survey of the correlation between geomagnetic activity and the near-Earth solar wind and two of the planetary indices at a wide variety of timescales. Various combinations of interplanetary parameters are evaluated with careful allowance for the effects of data gaps in the interplanetary data. We show that the theoretical coupling, P¿, function first proposed by Vasyliunas et al. is superior at all timescales from 1-day to 1-year.
On the basis of an ionospheric definition of disturbed conditions independent of any causative mechanism, a feature-guided pattern recognition method reveals the dominant morphology of long-duration negative foF2 disturbances. A catalogue of negative disturbances lasting more than 24 hours is compiled from hourly foF2 data from 75 ionosonde stations and three solar cycles. Disturbances in each month and station are handled separately, and four local time intervals of disturbance commencement are considered. A median disturbance profile is produced only when a minimum occurrence probability holds. The time window under morphological investigation is selected such that nonsystematic features, precursor phenomena, and poststorm effects are not included in analysis. The disturbance patterns, first grouped according to major characteristic features and then fitted with simple mathematic functions, are described by a range of the normalized deviation of hourly foF2 to its corresponding monthly median and are provided to radio users along with their distribution in space and time. The present model is a nonconditional stand-alone model which may, in the event of an ionospheric disturbance at a certain location, predict its further development.
In New Zealand after World War 2 radar techniques were used in various investigations in geophysics and astronomy. Much local expertise had become available from defence laboratories, which had been set up in 1939 and eventually merged into the Radio Development Laboratory, disbanded in 1946. Wartime radar development had in turn been founded on pre-war research in radio propagation and ionospheric research which included the use of pulse ionosondes. Frequent support for the pre-war radio research in both Britain and New Zealand was given by Ernest Rutherford who, throughout his life, retained the interest from his own early researches in radio wave propagation. This paper is a brief survey of events from Rutherford's early experiments in 1894 to present-day research programmes.
The October 2003 geomagnetic storm (often called the Halloween storm) was one of the largest storms (as measured by Dst) yet recorded. The storm-induced synoptic-scale changes in the ionosphere's plasma content and density can be viewed through space weather maps created by objective analysis algorithms. For this study, these maps, which specify the electron density in altitude, latitude, and longitude, are created by the ionospheric data assimilation three dimensional (IDA3D), a three-dimensional variation algorithm of the ionospheric electron density. These maps, representing the average conditions in the ionosphere over a 15 min sampling time, show how dramatically the ionosphere changed during the Halloween storm. Following the southward turning of the interplanetary magnetic field, the dayside electron content is significantly reduced in the equatorial ionosphere between +/-18 magnetic latitude and is enhanced poleward of this latitude. This is the expected behavior when the equatorial fountain is enhanced by a strong penetration electric field. In addition, the electron content is significantly increased in the dayside midlatitude ionosphere, which corresponds to a storm-enhanced density (SED) plume. Above 40 magnetic latitude, the dayside plasma content is significantly reduced in the regions adjacent to the SED structure, which enhances the electron content gradient. Electron density maps in the altitude-magnetic latitude plane show an increase in the topside electron densities within an SED plume.
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.
We present a study of the geographic location of lightning affecting the ionospheric sporadic-E (Es) layer over the ionospheric monitoring station at Chilton, UK. Data from the UK Met Office's Arrival Time Difference (ATD) lightning detection system were used to locate lightning strokes in the vicinity of the ionospheric monitoring station. A superposed epoch study of this data has previously revealed an enhancement in the Es layer caused by lightning within 200km of Chilton. In the current paper, we use the same data to investigate the location of the lightning strokes which have the largest effect on the Es layer above Chilton. We find that there are several locations where the effect of lightning on the ionosphere is most significant statistically, each producing different ionospheric responses. We interpret this as evidence that there is more than one mechanism combining to produce the previously observed enhancement in the ionosphere.
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.
There is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.
We apply a numerical model of time-dependent ionospheric convection to two directly driven reconnection pulses during a 15-min interval of southward IMF on 26 November 2000. The model requires an input magnetopause reconnection rate variation, which is here derived from the observed variation in the upstream IMF clock angle, θ. The reconnection rate is mapped to an ionospheric merging gap, the MLT extent of which is inferred from the Doppler-shifted Lyman-¿ emission on newly opened field lines, as observed by the FUV instrument on the IMAGE spacecraft. The model is used to reproduce a variety of features observed during this event: SuperDARN observations of the ionospheric convection pattern and transpolar voltage; FUV observations of the growth of patches of newly opened flux; FUV and in situ observations of the location of the Open-Closed field line Boundary (OCB) and a cusp ion step. We adopt a clock angle dependence of the magnetopause reconnection electric field, mapped to the ionosphere, of the form Enosin4(θ/2) and estimate the peak value, Eno, by matching observed and modeled variations of both the latitude, ¿OCB, of the dayside OCB (as inferred from the equatorward edge of cusp proton emissions seen by FUV) and the transpolar voltage ¿PC (as derived using the mapped potential technique from SuperDARN HF radar data). This analysis also yields the time constant ¿OCB with which the open-closed boundary relaxes back toward its equilibrium configuration. For the case studied here, we find ¿OCB = 9.7 +/- 1.3 min, consistent with previous inferences from the observed response of ionospheric flow to southward turnings of the IMF. The analysis confirms quantitatively the concepts of ionospheric flow excitation on which the model is based and explains some otherwise anomalous features of the cusp precipitation morphology.
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
All planetary atmospheres respond to the enhanced x-rays and ultraviolet (UV) light emitted from the Sun during a flare. Yet only on Earth are observations so continuous that the consequences of these essentially unpredictable events can be measured reliably. Here, we report observations of solar flares, causing up to 200% enhancements to the ionosphere of Mars, as recorded by the Mars Global Surveyor in April 2001. Modeling the attitude dependence of these effects requires that relative enhancements in the soft x-ray fluxes far exceed those in the UV.
The ionospheric F2-layer parameter long-term trends are considered from the geomagnetic control concept and the greenhouse hypothesis points of view. It is stressed that long-term geomagnetic activity variations are crucial for ionosphere long-term trends, as they determine the basic natural pattern of foF2 and hmF2 long-term variations. The geomagnetic activity effects should be removed from the analyzed data to obtain real trends in ionospheric parameters, but this is not usually done. Only a thermosphere cooling, which is accepted as an explanation for the neutral density decrease, cannot be reconciled with negative foF2 trends revealed for the same period. A more pronounced decrease of the O/N2 ratio is required which is not provided by empirical thermospheric models. Thermospheric cooling practically cannot be seen in foF2 trends, due to a weak NmF2 dependence on neutral temperature; therefore, foF2 trends are mainly controlled by geomagnetic activity long-term variations. Long-term hmF2 variations are also controlled by geomagnetic activity variations, as both parameters, NmF2 and hmF2 are related by the F2-layer formation mechanism. But hmF2 is very sensitive to neutral temperature changes, so strongly damped hmF2 long-term variations observed at Slough after 1972 may be considered as a direct manifestation of the thermosphere cooling. Earlier revealed negative hmF2 trends in western Europe, where magnetic declination D<0 and positive trends at the eastern stations (D>0), can be related to westward thermospheric wind whose role has been enhanced due to a competition between the thermosphere cooling (CO2 increase) and its heating under increasing geomagnetic activity after the end of the 1960s.
Using a numerical implementation of the cowlock92 model of flow excitation in the magnetosphere-ionosphere (MI) system, we show that both an expanding (on a 12-min timescale) and a quasi-instantaneous response in ionospheric convection to the onset of magnetopause reconnection can be accommodated by the Cowley-Lockwood conceptual framework. This model has a key feature of time dependence, necessarily considering the history of the coupled MI system. We show that a residual flow, driven by prior magnetopause reconnection, can produce a quasi-instantaneous global ionospheric convection response; perturbations from an equilibrium state may also be present from tail reconnection, which will superpose constructively to give a similar effect. On the other hand, when the MI system is relatively free of pre-existing flow, we can most clearly see the expanding nature of the response. As the open-closed field line boundary will frequently be in motion from such prior reconnection (both at the dayside magnetopause and in the cross-tail current sheet), it is expected that there will usually be some level of combined response to dayside reconnection.
This paper is the second in a series on a study of the link between IMF and sporadic-E layers within the polar cap. In Paper I (Voiculescu et al., 2006), an analysis of the sporadic-E data from Thule and Longyearbyen was presented. Here we concentrate on the electric field mechanism of sporadic-E generation. By means of model calculations we show that the mechanism is effective even at Thule, where the direction of the geomagnetic field departs from vertical only by 4. The model calculations also lead to a revision of the electric field theory. Previously, a thin layer was assumed to grow at a convergent null in the vertical ion velocity, which is formed when the electric field points in the NW sector. Our calculations indicate that in the dynamic process of vertical plasma compression, a layer is generated at altitudes of high vertical convergence rather than at a null. Consequently, the layer generation is less sensitive than previously assumed to fluctuations of the electric field direction within the NW sector. The observed diurnal variations of sporadic-E occurrence at Longyearbyen and Thule are compared with the diurnal variations of the electric field, calculated using a representative range of IMF values by means of the statistical APL model. The results indicate that the main features of Es occurrence can be explained by the convection pattern controlled by the IMF. Electric fields calculated from the IMF observations are also used for producing distributions of sporadic-E occurrence as a function of electric field direction at the two sites. A marked difference between the distributions at Thule and Longyearbyen is found. A model estimate of the occurrence probability as a function of electric field direction is developed and a reasonable agreement between the model and the experimental occurrence is found. The calculation explains the differences between the distributions at the two sites in terms of the polar cap convection pattern. The conclusion is that the electric field is the major cause for sporadic-E generation and, consequently, IMF has a clear control on the occurrence of sporadic E within the polar cap.
The meridional propagation velocities of the ionospheric F2-region response to 268 geomagnetic storms are calculated. Ionospheric vertical sounding data of I h time resolution from several stations located in a longitude sector approximately centred along the great circle that contains both the geomagnetic poles and the geographic poles are used.Most meridional propagation velocities from high to low latitudes are less than 600 m/s. The smaller velocities are typical of global neutral meridional wind circulation and the larger are representative of traveling atmospheric disturbances.
Simultaneous disturbances at several locations are more frequent during positive phases than during negative phases. Negative phase meridional propagation velocities associated with meridional neutral winds are less frequent in the southern hemisphere when compared with corresponding velocities observed in the northern hemisphere. This may be related to the fact that the distance between the geomagnetic pole and the equator is smaller in the northern hemisphere.
Most negative phase onsets are within the 06-10 LT interval. For middle geomagnetic latitudes a forbidden time interval between 11 and 14 LT is present. The positive phase onsets show the dusk effect. (c) 2005 Elsevier Ltd. All rights reserved.
The intense level of solar activity recorded from 16 to 23 January 2005 led to a series of events with different signatures at the Earth's ionospheric distances. Measurements of the critical frequency of the F 2 layer f o F 2 and the vertical total electron content (VTEC) are used to describe the temporal and spatial electron density distributions during this space weather event, which gives an excellent opportunity to test regional VTEC maps over Europe under such disturbed solar-terrestrial conditions. In this context, the tests used to validate the International GNSS Service (IGS) VTEC maps have been applied to assess the accuracy of the European Rutherford Appleton Laboratory (RAL) VTEC maps. Thus the self-consistency test and the Jason altimeter test have been used to compare such performances with the IGS and Universitat Politecnica de Catalunya global ionospheric maps. The results show discrepancies between the RAL maps and the IGS ones, which leads to significant RMS and bias values of several total electron content units. Moreover, in this work a kriging technique to improve the accuracy of any regional VTEC map is also considered, with relative improvements of the RAL VTEC maps up to more than 20% at the peak of the storm.
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.
Adding together the northern and southern hemisphere values for pairs of stations, the combined peak electron density NmF2 is greater in December-January than in June-July. The same applies to the total height-integrated electron content. This F2-layer annual asymmetry between northern and southern solstices is typically 30 exceeds the 7 Sun-Earth distance. Though it was noticed in ionospheric data almost seventy years ago, the asymmetry is still unexplained.Using ionosonde data and also values derived from the International Reference Ionosphere, we show that the asymmetry exists at noon and at midnight, at all latitudes from equatorial to sub-auroral, and tends to be greater at solar minimum than solar maximum. We find a similar asymmetry in neutral composition in the MSIS model of the thermosphere. Numerical computations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP) model give a much smaller annual asymmetry in electron density and neutral composition than is observed. Including mesospheric tides in the model makes little difference. After considering possible explanations, which do not account for the asymmetry, we are left with the conclusion that dynamical influences of the lower atmosphere (below about 30 km), not included in our computations, are the most likely cause of the asymmetry.
We use combinations of geomagnetic indices, based on both variation range and hourly means, to derive the solar wind flow speed, the interplanetary magnetic field strength at 1 AU and the total open solar flux between 1895 and the present. We analyze the effects of the regression procedure and geomagnetic indices used by adopting four analysis methods. These give a mean interplanetary magnetic field strength increase of 45.1 +/- 4.5 solar wind speed. We use averaging timescales of 1 and 2 days to allow for the difference between the magnetic fluxes threading the coronal source surface and the heliocentric sphere at 1 AU. The largest uncertainties originate from the choice of regression procedure: the average of all eight estimates of the rise in open solar flux is 73.0 +/- 5.0 procedure, giving the narrowest and most symmetric distribution of fit residuals, yields 87.3 +/- 3.9%.
Observations of HF radar backscatter from artificial field-aligned irregularities in an ionosphere perturbed by travelling disturbances due to atmospheric gravity waves are presented. Some features of the spatio-temporal structure of the artificial radar backscatter can be explained in terms of the distortion of the ionosphere resulting from the travelling disturbances. The distorted ionosphere can allow the HF pump wave to access upper-hybrid resonance at larger distances from the transmitter than are normally observed and can also prevent the pump wave reaching this resonance at close distances. The variation in altitude of the irregularities sometimes results in a significant variation in the elevation angle of arrival of the backscattered signal at the radar implying that the radar sees a target moving in altitude. We suggest that this may be evidence of off-orthogonal scattering from the irregularities.
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
n this paper we investigate the relationship between polar cap sporadic-E layers and the direction of the interplanetary magnetic field (IMF) using a 2-year database from Longyearbyen (75.2 CGM Lat, Svalbard) and Thule (85.4 CGM Lat, Greenland). It is found that the MLT distributions of sporadic-E occurrence are different at the two stations, but both are related to the IMF orientation. This relationship, however, changes from the centre of the polar cap to its border. Layers are more frequent during positive By at both stations. This effect is particularly strong in the central polar cap at Thule, where a weak effect associated with Bz is also observed, with positive Bz correlating with a higher occurrence of Es. Close to the polar cap boundary, at Longyearbyen, the By effect is weaker than at Thule. On the other hand, Bz plays there an equally important role as By, with negative Bz correlating with the Es occurrence. Since Es layers can be created by electric fields at high latitudes, a possible explanation for the observations is that the layers are produced by the polar cap electric field controlled by the IMF. Using electric field estimates calculated by means of the statistical APL convection model from IMF observations, we find that the diurnal distributions of sporadic-E occurrence can generally be explained in terms of the electric field mechanism. However, other factors must be considered to explain why more layers occur during positive than during negative By and why the Bz dependence of layer occurrence in the central polar cap is different from that at the polar cap boundary.
The validity of a technique developed by the authors to identify historical occurrences of intense geomagnetic storms, which is based on finding approximately coincident observations of sunspots and aurorae recorded in East Asian histories, is corroborated using more modern sunspot and auroral observations. Scientific observations of aurorae in Japan during the interval 1957-2004 are used to identify geomagnetic storms that are sufficiently intense to produce auroral displays at low geomagnetic latitudes. By examining white-light images of the Sun obtained by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the Debrecen Heliophysical Observatory and the Solar and Heliospheric Observatory spacecraft, it is found that a sunspot large enough to be seen with the unaided eye by an experienced observer was located reasonably close to the central solar meridian immediately before all but one of the 30 distinct Japanese auroral events, which represents a 97 Even an average observer would probably have been able to see a sunspot with the unaided eye before 24 of these 30 events, which represents an 80 success rate. This corroboration of the validity of the technique used to identify historical occurences of intense geomagnetic storms is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual historical geomagnetic storms during the greater part of the past two millennia.
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