Bi‐directional electron pitch angle anisotropy in the plasma sheet

Abstract

Analysis of low‐energy particle measurement on Imp 6 satellite have revealed the occurrence of bi‐directional electron pitch angle anisotropy (enhanced flux in field‐aligned directions) in the magnetotail at the energy range of several hundreds to several keV. The bi‐directional anisotropy was observed most commonly near the neutral sheet, with a parallel and perpendicular temperature ratio of 2–3, and they appeared also just inside of the magnetopause and in the boundary layer. The anisotropy of this extent was not an unusual phenomenon in the plasma sheet; in about 10% of the total of ∼200‐hour interval that we examined electrons had bi‐directional anisotropy. On the other hand, more sharply field‐aligned electron flux enhancements, which cannot be represented by a Maxwellian distribution, are sometimes observed at radial distances of ∼10 RE, near the inner edge of the plasma sheet. The small pitch angle (less than ∼10°) observed in such cases suggests coupling of the enhanced field‐aligned component with the ionosphere. In this paper we describe the observed characteristics of these anisotropic distributions and attempt to explain their formation by Fermi acceleration process in the plasma sheet. Owing to this process, electrons are more accelerated in parallel than perpendicular directions to the magnetic field as they traverse the neutral sheet. The effect of static electric field along the field line caused by the difference in pitch angle anisotropy of protons and electrons is also discussed.