An investigation on plasmaspheric electron content derived from ISR and GPS observations at Millstone Hill

Abstract

An attempt is made to estimate the plasmaspheric electron content (PTEC) using the data observed from the ISR (Incoherent Scatter Radar) and GPS at Millstone Hill (42. 6 degrees N, 288. 5 degrees E). We first present a brief description on the method we used in this study, then the variation of PTEC is studied. The electron density profile obtained by the ISR at Millstone Hill is fitted using Chapman function with varying scale heights, and then the ISR TEC is calculated as an integral of electron density from 100km to 1000km of the profile. The difference between GPS TEC (which is up to an altitude of 20200 km) and ISR TEC can be taken approximately as the plasmaspheric electron content (PTEC). Data from both high solar activity years (2000 and 2002) and low solar activity years (2005 and 2008) are used for the present study. The results we obtained indicate that both PTEC and the relative contribution of PTEC (to GPSTEC) exhibit obvious diurnal variation patterns, with PTEC being higher during daytime than during nighttime and the relative contribution of PTEC is higher during nighttime than during daytime. PTEC can vary from 4 to 14 TECU (1TECU = 10(16)el/m(2)) and the contribution of PTEC to GPS TEC reaches 60% during nighttime in the month of high solar activity level, while in the month of low solar activity, the value of PTEC is generally 3 similar to 7 TECU with the highest contribution about 80% at night. The variation pattern and trends in our results are in agreement with those obtained by other researches based on different measurements and techniques, and the magnitude is also agreed. This provides a support to the validity of our method. Unlike the ionosonde/digisonde which can only explore the bottomside ionosphere under the F2 peak, the ISR has the advantage to explore from a low to high altitude range and thus can provide both the bottomside and topside electron density profile. For this reason, the PTEC derived from ISR data (combined with GPS TEC) is more realistic than that from ionosonde data (combined with GPS TEC). This latter approach was often adopted by some former researchers.