Substorm-associated energetic ion (E ≅ 45 keV) flows at the plasma sheet boundary layer: A dawn-dusk flow reversal

Abstract

This work presents statistical results of ISEE-1 observations of energetic ions (E ≅ 45 keV) streaming at the plasma sheet boundary layer (PSBL), during thinnings and expansions of the tail plasma sheet associated with substorms. It is shown that the recovery PSBL events are more anisotropic, with higher peak intensities and shorter time durations than the thinning ones. Earthward streaming is found to be prevalent in the pre-midnight PSBL region, while tailward streaming is more frequent in the post-midnight flank. This flow reversal in the dominant streaming direction is more distinct for the entry events and is most probably the result of an underlying circulation pattern of energetic ions in the magnetotail. It is suggested that dawnward drifts of earthward-injected energetic ions, which take place in the presence of strong magnetic field gradients in the vicinity of the PSBL, play an important role in the generation of the circulation pattern and in particular the distinct tailward streaming at the dawn PSBL.