Annals 48, 3, 2005+app1 497 ANNALS OF GEOPHYSICS, VOL. 48, N. 3, June 2005 Key words topside ionosphere – electron density models – topside soundings 1. Introduction Information on the topside electron concen- tration distribution is not obtainable from ground- based measurements. In the past decades few satellites were equipped with ionosondes used for ionosphere sounding above the F2 maximum (topside). Moreover, only a few percent of data recorded in the 60s and 70s were processed at that time due to the limited techniques available for automatic ionogram scaling. However, re- cently large amounts of newly scaled ionograms have been made available on-line (Bilitza et al., 2003). This kind of data is particularly important for ionospheric modeling purposes, since ionos- pheric electron density models have their topside formulation based on old databases or different kind of measurements. Experimental vertical pro- files are very important for model comparison, because they provide the spatial distribution of the electron concentration and they contain the information on foF2, which is generally used by models as an anchor point to describe vertical profiles. The present work analyzes the behavior of the NeQuick and IRI models, adopted by In- ternational Telecommunication Union, Radio- communication sector (ITU-R) Recommenda- tion P. 531-6 (now superseded by P. 531-7) (ITU, 2001), with respect to the topside electron densi- ty profiles available in the databases of the ISIS2, IK19 and Cosmos 1809 satellites. This work is part of the Italian degree thesis in physics thesis of one author (Coïsson, 2002). 1.1. NeQuick model NeQuick model is an ionospheric electron density model, based on the original DGR «pro- Ionospheric topside models compared with experimental electron density profiles Pierdavide Coïsson and Sandro M. Radicella The Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy Abstract Recently an increasing number of topside electron density profiles has been made available to the scientific com- munity on the Internet. These data are important for ionospheric modeling purposes, since the experimental in- formation on the electron density above the ionosphere maximum of ionization is very scarce. The present work compares NeQuick and IRI models with the topside electron density profiles available in the databases of the ISIS2, IK19 and Cosmos 1809 satellites. Experimental electron content from the F2 peak up to satellite height and electron densities at fixed heights above the peak have been compared under a wide range of different con- ditions. The analysis performed points out the behavior of the models and the improvements needed to be as- sessed to have a better reproduction of the experimental results. NeQuick topside is a modified Epstein layer, with thickness parameter determined by an empirical relation. It appears that its performance is strongly affect- ed by this parameter, indicating the need for improvements of its formulation. IRI topside is based on Booker’s approach to consider two parts with constant height gradients. It appears that this formulation leads to an over- estimation of the electron density in the upper part of the profiles, and overestimation of TEC. Mailing address: Dr. Pierdavide Coïsson, The Abdus Sa- lam International Centre for Theoretical Physics (ICTP), Stra- da Costiera 11, 34014 Trieste, Italy; e-mail: coissonp@ictp.it 498 Pierdavide Coïsson and Sandro M. Radicella filer» (Di Giovanni and Radicella, 1990). It is a quick-run model for trans-ionospheric appli- cations that enables calculation of both vertical or slant electron density profile and TEC for any specified path (Hochegger et al., 2000; Radicella and Leitinger, 2001; Leitinger et al., 2002). Above 100 km and up to the F2-layer peak this model uses a modified DGR profile formulation, which includes five semi-Epstein layers with modeled thickness parameters and is based on anchor points defined by foE, foF1, foF2 and M(3000)F2 values. The topside is given by a semi-Epstein layer with a height de- pendent thickness parameter empirically deter- mined. The model has been adopted for assessment studies by the European Space Agency Euro- pean Geostationary Navigation Overlay Service (EGNOS) project and more recently by ITU-R (ITU, 2001) as a suitable method for TEC mod- eling. The standard NeQuick source code is available at . 1.2. IRI model The International Reference Ionosphere (IRI) is a well known and widely used empirical mod- el of the ionosphere (Bilitza 1990, 2001). For a given location, date and time, IRI describes the electron concentration, electron temperature, ion temperature and ion composition in the altitude range from about 50 km to 2000 km, as well as the TEC. IRI provides monthly averages in the non-auroral ionosphere for magnetically quiet conditions. It is periodically updated and has evolved over a number of years. IRI model can be run on-line at or the source code can be retrieved via anony- mous ftp from the NSSDC (National Space Sci- ence Data Center) site . 2. Experimental data used The databases of topside electron density profiles from the International Satellite for Ionos- phere Studies 2 (ISIS2) (ftp://nssdcftp.gsfc.na- sa.gov/spacecraft_data/isis/topside_sounder), In- tercosmos 19 (IK19) and Cosmos 1809 satellites (http://antares.izmiran.rssi.ru/projects/IK19) were used for model comparisons. The better- quality experimental data were selected for mod- el comparisons. The geographical distributions of the selected profiles are shown in fig. 1. The ISIS2 satellite was in a circular orbit at 1400 km height and available soundings were recorded from 1972 to 1983; the IK19 satellite was in an elliptical orbit between 500 km and 980 km height, with available data recorded in 1979 and 1980. Cosmos 1809 was in a circular orbit at 980 km height and data used were recorded during 1987. The NeQuick and IRI models were used to compute modeled topside profiles correspon- ding to the experimental ones; the peak values (foF2 and hmF2) obtained from the topside ex- perimental profiles were provided as input to the models, to focus the comparison on the shape of the topside profile. Other inputs given, which affect the modeled profile shape, are ge- ographic location, universal time and solar ac- tivity, expressed by R12. Figure 2 shows some example of experimental and modeled topside profiles for ISIS2 and IK19 satellites. 3. Results Since there are no universally accepted crite- ria to compare an experimental profile with a modeled one, two quantities were considered in this work: Total Electron Content (TEC) differ- ence from hmF2 up to the satellite height; elec- tron density ratio, Nmodel/Nexp, at a fixed height above the peak. For statistical purposes, the data have been divided into homogeneous groups of: solar ac- tivity level, season of the year, modip and local time of the measurement. Median values and the upper and lower quartiles of the distributions in each sector were computed. Data recorded by different satellites were kept separated, but the results show consistent behavior. In the statistical analysis performed the mod- ified dip (modip) coordinate was used. Modip µ is defined by Fig. 2. Comparison between experimental profiles from ISIS2 and IK19 data and NeQuick and IRI models: ex- perimental (X) solid black, NeQuick (N) dash-dotted light gray, IRI (I) dashed dark gray. TEC in 1016 m−2. 499 Ionospheric topside models compared with experimental electron density profiles Fig. 1. Locations of the available profiles: (top panel) ISIS2 data, names indicate the telemetry stations used; (bottom panel) IK19 data. 500 Pierdavide Coïsson and Sandro M. Radicella tan cos /1 2=n z } ^ h (3.1) ψ being the true magnetic dip in the ionosphere (usually at 300 km) and φ the geographic latitude (Rawer, 1963). Data were divided into modip in- tervals to cover the different regions: equatorial region (−5°, 5°), anomaly regions (−35°, −5°) and (5°, 35°), mid-latitudes (−60°, −35°) and (35°, 60°) and high-latitudes (−90°, −60°) and (60°, 90°). Months were analyzed in three groups: first group from November to February, second group from May to August, third group March, April, September and October. Various local time intervals were considered: day from 09:00 to 17:00 LT, night from 21:00 to 05:00 LT, sunrise from 05:00 to 07:00 LT, sunset from 18:00 to 20:00 LT. For low solar activity R12 < 40 was considered and for high solar activity R12 > 100. 3.1. TEC comparisons The percent differences between each mod- eled and experimental TEC value integrated from hmF2 up to the satellite height were cal- culated and a statistical analysis performed as indicated in the previous section, the most im- portant results are given below. NeQuick model – Figure 3 shows a different behavior in the first and second group of months. During the first one it overestimates and underestimates the TEC anti-symmetrically with respect to the magnetic equator. During the second one it shows a symmetric behavior, with overestimation at low latitudes and better agreement at medium and high latitudes. These differences are associated with the different for- mulation of the NeQuick topside thickness pa- rameter in the different months (Radicella and Zhang, 1995). IRI model – Figure 4 shows that IRI tends to overestimate the topside TEC, with a behavior dependent on solar activity. While during low solar activity there is a greater overestimation at low latitudes and a good agreement at medium latitudes, for high solar activity IRI underesti- mates TEC in the equatorial region but strong- ly overestimates it at high latitudes. Day and night behaviors are similar in trend but there is better agreement at low latitudes. Fig. 3. Percent difference between experimental topside TEC and NeQuick topside TEC for months group 1 and 2. Numbers indicate the amount of data in each section. 501 Ionospheric topside models compared with experimental electron density profiles Fig. 4. Comparison of topside TEC, IRI model: percent difference for months groups 1 and 2. Numbers indi- cate the amount of data. Fig. 5. Comparison of electron density, NeQuick model, at 900 km above hmF2 for months groups 1 and 2. Numbers indicate the amount of data. 3.2. Electron density comparisons The experimental and modeled electron den- sities 900 km above hmF2 were compared. This height was reached only in the ISIS2 profiles, but it was chosen to assess models behavior in the upper part of the profile: the IRI model is used up to 2000 km and NeQuick to 20000 km. 502 Pierdavide Coïsson and Sandro M. Radicella Fig. 6. Comparison of electron density, IRI model, 900 km above hmF2 for months groups 1 and 2. Numbers indicate the amount of data. Thus it is important to understand how they are reliable in that region. Here results are shown only for local day time, because the number of data available for local night was not enough to provide good statistics in all regions. The small amount of night-time profiles confirms the re- sults shown for day-time, the only relevant dif- ference is indicated in the following description. NeQuick model – Figure 5 shows a seasonal behavior similar to the one observed for topside TEC, with the electron density values contained between 0.2 and 3 times the experimental ones: little underestimation or overestimation. The be- havior during day-time and night-time is simi- lar, only for low solar activity can a difference be noticed at low modip, where the model goes from low underestimation to low overestimation and vice versa depending on month. IRI model – Figure 6 demonstrates that IRI always overestimates the electron density; the median values are greater than 1.5 times the ex- perimental ones and can reach up to 9 times. It shows a different modip behavior depending on solar activity level: during low solar activity the greater discrepancy is found at low modip, while for high solar activity the situation is re- versed. At high latitudes for high solar activity cases have been found in which the IRI electron density does not decrease with height. 4. Conclusions Comparisons have been made between 21000 topside electron density profiles from satellite measurements and those computed by the NeQuick and IRI models. Attention was fo- cused on TEC and electron density at 900 km above the F2 peak. The wide set of different ex- perimental conditions has enabled assessment of many characteristics of each model. The NeQuick topside is a modified Epstein layer. Its thickness parameter is governed by an empirical relation, with a dependence related to the month of the year. From this analysis it ap- pears that the model performance is strongly af- fected by this parameter, indicating the need for an improvement of its formulation. IRI is based on Booker’s approach to mod- el the topside by dividing it into two parts with 503 Ionospheric topside models compared with experimental electron density profiles constant height gradients. The gradients depend on latitude, solar activity and foF2. In the analy- sis carried out it appears that this formulation leads to overestimation of the electron density in the upper part of the profiles, and consequently to overestimation of TEC. REFERENCES BILITZA, D. (1990): International Reference Ionosphere 1990, NSSDC 90-22, Greenbelt, Maryland. 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