Topside plasma scale height retrieved from radio occultation measurements
Introduction
One of the most important characteristics of the ionosphere–plasmasphere system is the plasma scale height, defined as HP = kTP/mig, where TP is the plasma temperature, Ti and Te are the ion and electron temperatures, k is the Boltzmann’s constant, mi is the ion mass, g is the acceleration due to gravity. Practically, HP is the vertical distance in which the plasma concentration changes by a factor of an exponent (e 2.718281828). Given its dependence on the plasma temperature and composition, both varying with altitude (Figs. 1 panel A and panel B), the plasma scale height value will obviously also vary with altitude (Fig. 1 panel C). The initial assessment of the topside ionosphere scale height and its intrinsic connection with the topside electron density profile (EDP) is of crucial importance for the reliability of various GPS TEC calculation and plasma density reconstruction techniques (Heise et al., 2002, Stankov et al., 2003). The CHAMP satellite (Reigber et al., 2003), orbiting the Earth in the altitude region from 450 down to about 300 km, provides excellent opportunities for observation of the topside ionosphere on a global scale, including the plasma scale height.
First results from the retrieval and analysis of the topside plasma scale height behaviour, based on CHAMP ionospheric radio occultation (IRO) measurements (Jakowski et al., 2002), are presented here. Latitudinal, seasonal, and diurnal variations have been already obtained. Considering the increasing number of low-earth-orbiting (LEO) satellites and the growing IRO measurement data base, it will be possible to develop a new empirical model of the topside plasma scale height to be used in several applications, such as improving the IRO-based EDP retrieval or various ionosphere–plasmasphere composition and dynamics.
Section snippets
Scale height retrieval
The GPS ionospheric radio occultation (IRO) measurements, carried out onboard CHAMP, started on 11 April 2001. The dual frequency carrier phases of the GPS signals are used to compute the total electron content (TEC) along the 1 Hz sampled occultation ray paths. To enable consideration of the horizontal gradients, particularly during ionospheric storms and/or near the crest region, a tomographic approach (Jakowski et al., 2002) is utilised for retrieving the vertical refractive index profile
Scale height behaviour
Using observations from one full year (April 2002–March 2003), an important insight into the plasma scale height behaviour has been acquired for various local time, season, and latitudinal conditions. During this period, the solar activity is on descent – the F10.7 monthly average falling from about 200 down to 120. Day-time and night-time conditions are presented. To minimise the dawn/dusk effects we have used data from variable local time ranges (windows) depending on season: larger windows
Summary and conclusions
We presented first results from the retrieval and alalysis of the topside plasma scale height using IRO measurements onboard CHAMP. It has been shown that it is possible to utilise these observations for acquiring a valuable knowledge of the plasma scale height behaviour. First and foremost, it has been proven that the topside plasma scale height depends strongly on the ionosphere–plasmasphere temperature, composition and dynamics. Second, it is clear that the scale height generally increases
Acknowledgements
This research is funded by the German Aerospace Centre (DLR), the German State Government of Mecklenburg-Vorpommern under grant V230-630-08-TIFA-334, and by the NATO Science Program under grant EST-CLG-979784.
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