Topside plasma scale height retrieved from radio occultation measurements

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Abstract

The plasma scale height is one of the important ionospheric characteristics due to its intrinsic connection to the ionospheric plasma temperature and composition, and thus to the shape of the electron density profile. Therefore, the knowledge of the plasma scale height values and variation is of crucial importance when addressing several open scientific and technological questions such as the upper ionosphere temperature balance, ion composition, storm dynamics, etc. While the plasma scale height value in the bottomside ionosphere can be deduced directly and reliably enough by vertical incidence sounding, the plasma scale height in the topside ionosphere is difficult to obtain. The ionospheric radio occultation (IRO) technique, based on low-earth-orbiting (LEO) satellites, is capable of delivering valuable information on the topside plasma scale height behaviour and of providing a rich database for consequent development of new empirical models of the plasma scale height and density. The purpose of this paper is to present a new procedure for retrieval of the topside plasma scale height value from IRO measurements by the LEO satellite CHAMP. First results from the analysis of the topside plasma scale height’s temporal and spatial variations are also provided.

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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