Description of the mean behaviour of ionospheric plasma temperatures

A status report on the empirical modeling of ionospheric electron and ion temperatures is given with special emphasis on the models used in the International Reference Ionosphere (IRI).Electron temperature models have now reached a state where reliable prediction of the mean altitudinal, latitudinal and diurnal variations is possible. These models are largely based on satellite measurements, but comparisons with incoherent scatter radar measurements have shown excellent agreement. Variations with season and magnetic and solar activity seem to be small and are not yet included consistently in these models.Similar to the electron temperature, the ion temperature shows the largest variations with altitude, latitude and local time. But due to the larger mass, these variations are smoother and more steady in the case of the ions and therefore easier to model. Nevertheless, very few ion temperature models exist. The IRI model takes advantage of the observed concurrence of the ion temperature with the neutral temperature at low altitudes and with the electron temperature at high altitudes.     

Electric field measurements at a southern mid-latitude station obtained using an HF digital ionosonde

It is important to understand the convection of the inner magnetosphere to fully describe the response of the low- to mid-latitude thermosphere-ionosphere system to geomagnetic storms. Realistic numerical simulations of mid-latitude electric fields suffer from limited knowledge of lower thermospheric winds and ionospheric conductivity on a global scale. Even empirical models of mid-latitude electric fields suffer from the paucity of measurements made by the handful of incoherent scatter radars concentrated in the American-European sector, and the intermittent satellite measurements made in other regions. Thus it would be very useful to show the extent to which Doppler velocity measurements made with the numerous digital ionosondes deployed around the globe can be used to infer F-region electric fields. The monthly average diurnal variation of Doppler velocity measured by a recently commissioned Digisonde at Bundoora (145.1°E, 37.7°S, geographic; 49°S magnetic) is seen to resemble the average diurnal variation of ion drift measured by the incoherent scatter radar at Millstone Hill (71.5°W 42.6°N; 57°N). Moreover, the Bundoora measurements exhibit the nighttime westward perturbation drifts found in Dynamics Explorer-2 ion drift measurements.     

Equatorial anomaly according to the Interkosmos-19 data and IRI model: A comparison

The comparison of the IRI model with the foF2 distribution in the equatorial anomaly region obtained by topside sounding onboard the Interkosmos-19 satellite has been carried out. The global distribution of foF2 in terms of LT-maps was constructed by averaging Intercosmos-19 data for summer, winter, and equinox. These maps, in fact, represent an empirical model of the equatorial anomaly for high solar activity F10.7 ~ 200. The comparison is carried out for the latitudinal foF2 profiles in the characteristic longitudinal sectors of 30, 90, 210, 270, and 330°, as well as for the longitudinal variations in foF2 over the equator. The largest difference between the models (up to 60%) for any season was found in the Pacific longitudinal sector of 210°, where there are a few ground-based sounding stations. Considerable discrepancies, however, are sometimes observed in the longitudinal sectors, where there are many ground-based stations, for example, in the European or Indian sector. The discrepancies reach their maximum at 00 LT, since a decay of the equatorial anomaly begins before midnight in the IRI model and after midnight according to the Interkosmos-19 data. The discrepancies are also large in the morning at 06 LT, since in the IRI model, the foF2 growth begins long before sunrise. In the longitudinal variations in foF2 over the equator at noon, according to the satellite data, four harmonics are distinguished in the June solstice and at the equinox, and three harmonics in the December solstice, while in the IRI model only two and one harmonics respectively are revealed. In diurnal variations in foF2 and, accordingly, in the equatorial anomaly intensity, the IRI model does not adequately reproduce even the main, evening extremum.     

Assessment of improvement of the IRI model over Ethiopia for the modeling of the variability of TEC during the period 2013–2016

This paper discusses the monthly and seasonal variation of the total electron content (TEC) and the improvement of performance of the IRI model in estimating TEC over Ethiopia during the solar maximum (2013–2016) phase employing as reference the GPS derived TEC data inferred from four GPS receivers installed in different regions of Ethiopia; Assosa (geog 10.05°N, 34.55°E, Geom. 7.01°N), Ambo (8.97°N, 37.86°E, Geom. 5.42°N), Nazret (8.57°N, 39.29°E, Geom. 4.81°N) and Arba Minch (6.06°N, 37.56°E, Geom. 2.62°N). The results reveal that, in the years 2013–2016, the highest peak GPS-derived diurnal VTEC is observed in the March equinox in 2015 over Arba Minch station. Moreover, both the arithmetic mean GPS-derived and modelled VTEC values, generally, show maximum and minimum values in the equinoctial and June solstice months, respectively in 2014–2015. However, in 2013, the minimum and maximum arithmetic mean GPS-derived values are observed in the March equinox and December solstice, respectively. The results also show that, even though overestimation of the modelled VTEC has been observed on most of the hours, all versions of the model are generally good to estimate both the monthly and seasonal diurnal hourly VTEC values, especially in the early morning hours (00:00–03:00?UT or 03:00–06:00?LT). However, it has also been shown that the IRI 2007 and IRI 2012 versions generally perform best in matching the diurnal GPS derived TEC values as compared to that of the IRI 2016 version. In addition, the IRI 2012 version with IRI2001 option for the topside electron density shows the highest overestimation of the VTEC as compared to the other options. None of the versions of the IRI model are proved to be able to capture the effects of geomagnetic storms.     

NeQuick 2 and IRI Plas VTEC predictions for low latitude and South American sector

Using vertical total electron content (VTEC) measurements obtained from GPS satellite signals the capability of the NeQuick 2 and IRI Plas models to predict VTEC over the low latitude and South American sector is analyzed. In the present work both models were used to calculate VTEC up to the height of GPS satellites. Also, comparisons between the performance of IRI Plas and IRI 2007 have been done. The data correspond to June solstice and September equinox 1999 (high solar activity) and they were obtained at nine stations. The considered latitude range extends from 18.4°N to ?64.7°N and the longitude ranges from 281.3°E to 295.9°E in the South American sector. The greatest discrepancies among model predictions and the measured VTEC are obtained at low latitudes stations placed in the equatorial anomaly region. Underestimations as strong as 40?TECU [1?TECU?=?10¹⁶?m^?2] can be observed at BOGT station for September equinox, when NeQuick2 model is used. The obtained results also show that: (a) for June solstice, in general the performance of IRI Plas for low latitude stations is better than that of NeQuick2 and, vice versa, for highest latitudes the performance of NeQuick2 is better than that of IRI Plas. For the stations TUCU and SANT both models have good performance; (b) for September equinox the performances of the models do not follow a clearly defined pattern as in the other season. However, it can be seen that for the region placed between the Northern peak and the valley of the equatorial anomaly, in general, the performance of IRI Plas is better than that of NeQuick2 for hours of maximum ionization. From TUCU to the South, the best TEC predictions are given by NeQuick2.The source of the observed deviations of the models has been explored in terms of CCIR foF2 determination in the available ionosonde stations in the region. Discrepancies can be also related to an unrealistic shape of the vertical electron density profile and or an erroneous prediction of the plasmaspheric contribution to the vertical total electron content. Moreover, the results of this study could be suggesting that in the case of NeQuick, the underestimation trend could be due to the lack of a proper plasmaspheric model in its topside representation. In contrast, the plasmaspheric model included in IRI, leads to clear overestimations of GPS derived TEC.     

Comparison of various empirical models of electron temperature with experimental measurements during low solar activity

Comparisons of various available empirical models of electron temperature are made with actual measurements from incoherent scatter radar and rocket and satellite probes, during low solar activity period. The models compared are those of Pandey $\text{et al.}$ (1983), Brace and Theis (1978), IRI (1979) and Bilitza (1983). It is found that our model and the Brace and Theis model are closer to actual measurements than the IRI and Bilitza models.     

Improved analytical representation of electron temperature in the IRI

In the last decade extensive measurements from incoherent scatter stations and from several satellite missions have considerably improved our knowledge of long- and short-term variations in the ionospheric electron temperature. Comparisons with IRI have revealed some shortcomings of this earlier model. It is obvious that in different altitude regions one has to concentrate the modelling efforts on different parameters. Here a model representation is proposed that will facilitate approaches (for the different altitude regions) in one analytic form.     

Modelling of ionospheric temperature profiles

The amount of measured temperature data accumulated in recent years allows and asks for improvement and refinement of the rather crude temperature models employed in the International Reference Ionosphere 1979. By combining the mission-oriented models by BRACE, THEIS /2/ for the AE-C satellites and by SPENNER, PLUGGE /1/ for the AEROS-A satellite, a much better diurnal and latitudinal reliability can be obtained. It is also suggested that IRI should have the option to make use of the strong anti-correlation between electron temperature and density in cases where actual measured densities are available. For daytime condition, incorporation of a density dependent model into IRI can significantly enhance the prediction quality of IRI in the altitude range 300 to 600 km.Furtheron, the solar activity dependence of the electron temperature and of the density-temperature-relation are investigated by comparing the low solar activity models with more recent AE-C and -DE data.     

Ionospheric behavior of the F2 peak parameters foF2 and hmF2 at Hainan and comparisons with IRI model predictions

Monthly median values of foF2, hmF2 and M(3000)F2 parameters, with hourly time interval resolution for the diurnal variation, obtained with DPS-4 digisonde observations at Hainan (19.4°N, 109.0°E) are used to study the low latitude ionospheric variation behavior. The observational results are compared with the International Reference Ionospheric Model (IRI) predictions. The time period coverage of the data used for the present study is from March 2002 to February 2005. Our present study showed that: (1) In general, IRI predictions using CCIR and URSI coefficients follow well the diurnal and seasonal variation patterns of the experimental values of foF2. However, CCIR foF2 and URSI foF2 IRI predictions systematically underestimate the observed results during most time period of the day, with the percentage difference ΔfoF2 (%) values changing between about −5% and −25%, whereas for a few hours around pre-sunrise, the IRI predictions generally overestimate the observational ones with ΔfoF2 (%) sometimes reaching as large as ∼30%. The agreement between the IRI results and the observational ones is better for the year 2002 than for the other years. The best agreement between the IRI results and the observational ones is obtained in summer when using URSI coefficients, with the seasonal average values of ΔfoF2 (%) being within the limits of ±10%. (2) In general, the IRI predicted hmF2 values using CCIR M(3000)F2 option shows a poor agreement with the observational results. However, when using the measured M(3000)F2 as input, the diurnal variation pattern of hmF2 given by IRI2001 has a much better agreement with the observational one with the detailed fine structures including the pre-sunrise and post-sunset peaks reproduced reasonably well. The agreement between the IRI predicted hmF2 values using CCIR M(30,000)F2 option and the observational ones is worst for the afternoon to post-midnight hours for the high solar activity year 2002. During daytime hours the agreement between the hmF2 values obtained with CCIR M(30,000)F2 option and the observational ones is best for summer season. The discrepancy between the observational hmF2 and that obtained with CCIR M(30,000)F2 option stem from the CCIR M(3000)F2 model, which does not produce the small scale structures observed in the measured M(3000)F2.     

Comparison between the IRI ion composition and incoherent scatter measurement and theoretical values

Comparisons have been made between the percentage of light ions in the upper ionosphere as predicted by the IRI model and as found in incoherent scatter (ICS) measurements at the stations Millstone Hill, Arecibo and Jicamarca. Major discrepancies are observed in both day and night. The IRI values are always considerably larger than the ICS measurements. Theoretical values are calculated as well, assuming chemical equilibrium and using the MSIS neutral density model /1/. In most cases these theoretical values favour the ICS values; only for the daytime ion composition above Millstone Hill has better agreement with the IRI model been found.     

An empirical model of electron temperature for low and middle latitudes in the 100–200 km height region

An empirical model of electron temperature (T_e) for low and middle latitudes is proposed in view of IRI. It is constructed on the basis of experimental data obtained at 100 to 200 km by probe and incoherent scatter methods. Below 150 km the model gives two T_e values: one from incoherent scatter data and another from probe measurements. The model can be used for all seasons for quiet geomagnetic conditions (Kp not greater 3) and at almost all levels of solar activity (F_10.7 between 70 and 200). It is presented in an analytical form that allows one to calculate T_e profiles for different latitudes, longitudes and at any season (day). Depending on geomagnetic latitude and solar zenith angle, electron temperature distributions are presented for two heights along with T_e profile variations during the day (at middle latitudes).     

Validate the IRI2007 model by the COSMIC slant TEC data during the extremely solar minimum of 2008

During 2008, the solar activity is extremely low. The satellite observations show that the ionospheric height and electron density is much lower than the predictions by the international reference ionosphere (IRI) model. In this paper, we compared the slant total electron content (TEC) observed by the COSMIC satellites during 2008 with the IRI model results. It is found that the IRI model with IRI2001 and IRI2001 Cor. topside options will always overestimate the electron density in both lower and higher altitudes. But the rest two topside options (NeQuick, and TTS) tend to overestimate the electron density in the F layer and underestimate it in the topside altitudes. The switch altitude between overestimation and underestimation and the latitude-local time distribution of the model deviation depend on the topside option. The current investigation might be useful for the model improvement as well as data assimilation work based on the IRI model and the LEO TEC data.     

Ionospheric variation at Thailand equatorial latitude station: Comparison between observations and IRI-2001 model predictions

In this paper, the F2-layer critical frequency (foF2) and peak height (hmF2) measured by the FM/CW ionosonde at Thailand equatorial latitude station, namely Chumphon (10.72°N, 99.37°E, dip 3.22) are presented. The measurement data during low solar activity from January 2004 to December 2006 are analyzed based on the diurnal, seasonal variation. The results are then compared with IRI-2001 model predictions. Our study shows that: (1) In general, both the URSI and CCIR options of the IRI model give foF2 close to the measured ones, but the CCIR option produces a smaller range of deviation than the URSI option. The agreement during daytime is generally better than during nighttime. Overestimation mostly occurs in 2004 and 2006, while underestimation is during pre-sunrise hours in June solstice in 2005. The peak foF2 around sunset is higher during March equinox and September equinox than the other seasons, with longer duration of maximum levels in March equinox than September equinox. Large coefficients of variability foF2 occur during pre-sunrise hours. Meanwhile, the best agreement between the observed foF2 and the IRI model is obtained in June solstice. (2) In general, The IRI (CCIR) model predicts the observed hmF2 well during daytime in June solstice from 2004–2006, but it overestimates during March equinox, September equinox and December solstice. For nighttime, the model overestimates hmF2 values for all seasons especially during March equinox and September equinox. However, the model underestimates hmF2 values during September equinox and for some cases during June solstice and December solstice at pre-sunrise. The agreement between the IRI model and the hmF2(M3000_OBS) is worst around noontime, post-sunset and pre-sunrise hours. All comparative studies give feedback for new improvements of CCIR and URSI IRI models.     

Testing the improvement of performance of the IRI model in the estimation of TEC over the mid-latitude American regions

This paper mainly discusses the improvement of performance of the International Reference Ionosphere (IRI) model in estimating the variation of the Vertical Total Electron Content (VTEC) over the mid latitude American regions during the relatively low (2008–2010) and relatively high (2012) solar activity years. This has been conducted employing the VTEC values obtained from the dual frequency ground based Global Positioning System (GPS) receivers located at Mineral Area Community College, MACC (37.85°N, 269.52°W) and Mississippi County Airport, MAIR (36.85°N, 270.64°W), and the latest versions of the IRI online model (IRI 2007, IRI 2012 and IRI 2016). The study mainly focuses to compare the trend of variability of the monthly and seasonal modeled VTEC values (IRI 2007 VTEC, IRI 2012 VTEC and IRI 2016 VTEC) with the corresponding measured VTEC values (GPS VTEC). The overall results show that the IRI VTEC values (almost in all versions of the model) are generally smaller than the GPS VTEC except after about 15:00 UT (09:00 LT) in the December solstice when the Sun shifts to the high solar activity. On the contrary, overestimations of the VTEC values by the model are observed in traversing from the low solar activity (2008) to high solar activity (2012) phase, especially after about 15:00 UT (09::00 LT) with the IRI 2016 version showing the highest. In general, the IRI 2007 and IRI 2012 versions show similar monthly and seasonal underestimations or overestimations showing that the two versions have almost similar performance. The IRI 2016 version is generally better in capturing both the diurnal and arithmetic mean GPS VTEC values with some exceptional months and seasons as compared to those of the IRI 2007 and IRI 2012 versions.     

基于COSMIC掩星精密定轨数据的等离子体层电子含量研究

等离子体层是日地环境重要的组成部分.本文利用COSMIC掩星精密定轨数据经处理后得到的podTec文件获取等离子体层电子含量(PEC)对等离子体层进行研究.将podTec数据进行处理后获得的PEC(pod-PEC)和IRI-Plas经验模型提供的PEC(IRI-PEC)进行对比,发现pod-PEC与IRI-PEC符合得...