The electron pitch angle distribution at geosynchronous orbit associated with absorption spikes during the substorm expansion phase

Abstract

Magnetospheric electron (16–300 keV) and ion (27–400 keV) fluxes associated with auroral abosrption events named ‘absorption spikes', have been investigated. The particle flux observations together with magnetic field measurements were made on board the geosynchronous satellite GEOS 2. Data from three consecutive substorm activations were analysed. The events were associated with strong variations of the electron fluxes, but only weak variations of the ion fluxes. We find strong indications that the electron pitch angle distributions associated with the absorption spike were characterized by maxima near 0°, 180°, and 90°, and dominated by a strong field aligned bi‐directional anisotropy at energies below 100 keV. At lower magnetic latitudes the field‐aligned anisotropy is decreased in magnitude and the flux maximum at 90° pitch angle.