Plasma electrons as tracers of distant magnetotail structure: ISEE-3

Abstract

Electrons in the 50–500 eV energy range commonly exhibit strong, field-aligned bidirectional anisotropies in the distant (r > 100 R E) geomagnetic tail lobes and are found to occur predominantly in the lobe directly connected to the sun along the interplanetary magnetic field in the open magnetosphere model (north lobe for away interplanetary sectors and south lobe for toward sectors). Data show the transition from unidirectional (sheath) electron populations to bidirectional (lobe) populations at the distant magnetopause. This demonstrates the open nature of the distant magnetotail and shows that the source of the higher-energy, bidirectional lobe electrons is the tailward-directed electron heat flux population in the magnetosheath. The field-aligned lobe electron phase space densities above ∼ 200 eV at ISEE-3 agree well with DMSP-measured polar rain phase space densities near the polar cap and the spectral slopes above 200 eV also are similar. Below 100–200 eV there is a thermal electron population in the distant tail, arising from local entry of plasma through the distant magnetopause, which is not present at DMSP altitudes. These data show that the suprathermal tail lobe electrons are essentially a test particle population which can move freely along field lines to form polar rain; in contrast, the thermal electrons are bound to the tailward-flowing lobe ion population far down the tail and thus cannot reach the polar cap regions.