Low energy charged particle observations in the "auroral" magnetosphere: First results from the Akebono (EXOS-D) satellite.

Abstract

The Japanese Akebono (EXOS-D) satellite was successfully put into a semipolar, elliptical orbit in order to study particle acceleration processes in the auroral magnetosphere. The onboard LEP (low energy particle) instrument makes three distinct types of charged particle observations: 1) energy and pitch-angle distributions of auroral electrons and ions, 2) mass per charge analysis of positive ions, and 3) onboard detection of particle flux modulations in the HF and VLF ranges. In this report we describe the LEP instrumentation, and present some preliminary results from item 1 above. Due to the initial orbital configuration, emphasis is placed on dayside phenomena, including the polar cusp. Unique examples of high particle activity in the polar cap and the hot plasma distribution along the transpolar arc are also presented. The observations in the cusp region reveal intense but patchy electron fluxes accompanied by keV-range ions with a characteristic energy versus time/space dispersion. The data are interpreted as signatures of longitudinal and/or temporal fluctuations of the magnetosheath plasma injection into the polar cusp. The dayside is characterized by the frequent appearance of low-energy ion conics and beams accompanied by intense low-energy electron precipitation. Finally it is noted that low-energy accelerated ions upgoing along the magnetic field line are observed in a very limited region around the dayside edge of the transpolar arc.