Banded electron structures in the plasmasphere

Abstract

The low‐energy plasma analyzer on CRRES has detected significant fluxes of 10‐eV to 30‐keV electrons trapped on plasmaspheric field lines. On energy‐versus‐time spectrograms these electrons appear as banded structures that can span the 2 < L < 6 range of magnetic shells. We present an example of banded electron structures, encountered in the nightside plasmasphere during the magnetically quiet January 30, 1991. Empirical analysis suggests that two clouds of low‐energy electrons were injected from the plasma sheet to L < 4 on January 26 and 27 while the convective electric field was elevated. The energies of electrons in the first cloud were greater than those in the second. DMSP F8 measurements show that after the second injection, the polar cap potential rapidly decreased from >50 to <20 kV. Subsequent encounters with the lower‐energy cloud on alternating CRRES orbits over the next 2 days showed a progressive, earthward movement of the electrons’ inner boundary. Whistler and electron cyclotron harmonic emissions accompanied the most intense manifestations of cloud electrons. The simplest explanation of these measurements is that after initial injection, the Alfvén boundary moved outward, leaving the cloud electrons on closed drift paths. Subsequent fluctuations of the convective electric field penetrated the plasmasphere, transporting cloud elements inward. The magnetic shell distribution of electron temperatures in one of the banded structures suggests that radiative energy losses may be comparable in magnitude to gains due to adiabatic compression.