Electron density near the plasmapause measured over one year by GEOS-2: a statistical analysis

This paper presents the results of a statistical analysis made under the 1 h average regime from the electron density data obtained over one year by the relaxation sounder on board the satellite GEOS-2 of ESA. The electron density diurnal variations, monthly and annually averaged, are sorted out. A comparison between monthly averaged, daily electron density profiles obtained over all the year has revealed the existence of a seasonal variation of plasma density in the equatorial region, the electron density being larger, on average, in summer than in winter. This seasonal variation is superimposed on the variation related to geomagnetic activity. The asymmetry of the Earth's internal magnetic field is mentioned as being a potential candidate for explaining this seasonal modulation. The annually averaged, daily profile is given. It is found to reach a maximum at 1600 LT, which indicates a plasmaspheric bulge in the predusk, rather than dusk, sector. This is found to be significantly earlier than the average LT location of the stagnation point (≳ 1800 LT) inferred from previous empirical studies for comparable geomagnetic activity levels. This latter feature is interpreted as resulting from the asymmetry of the individual daily density profiles with respect to their maximum, which has been previously reported, and whose effect, after averaging individual daily profiles having their maxima distributed around dusk, is to shift the maximum of the average profile toward the predusk sector.