High-energy electron detection onboard DEMETER: The IDP spectrometer, description and first results on the inner belt

This paper gives a description and the first results of the electron spectrometer placed onboard the Demeter satellite. This detector with a large geometrical factor is aimed to measure trapped electron fluxes in the energy range from 70 keV to about 0.8 MeV and to provide information on the electron fluxes between 0.8 and 2.5 MeV. The energy resolution, better than 10 keV, and the 256 energy channels allow to obtain insights on the radiation belt structure. The data received during 2004 show for the first time that the low-energy component of the inner radiation belt ( 1.1 < L < 1.35 ) consists of the superposition of quasi-mono-energetic peaks as expected from the interaction of electrons with a large number of high-powered Earth-based VLF emitters in the frequency range 10–100 kHz distributed along the particle longitudinal drift around the Earth. This energy-layered structure appears to depend on the longitude, being particularly evident in low magnetic field regions where particles from the inner belt can easily reach the satellite altitude. This happens locally West of America and inside the South Atlantic Anomaly itself. We show that the variation of the energy of the peaks as a function of the McIlwain parameter can be used to compute the altitude profile of the equatorial thermal electron content corresponding to the transition between the upper ionosphere and the plasmasphere.