Observations of polar rain at low and high altitudes

Abstract

Observations in the distant magnetotail lobes (r ≳ 100 RE) reveal that electrons in the ∼50‐ to 1000‐eV energy range often exhibit strong, field‐aligned bidirectional anisotropies. Because of large tail motions in the variable flow of the solar wind, ISEE 3 data provide extensive sampling of most tail plasma regions in rapid succession. It is found that bidirectional electron fluxes occur predominantly in the lobe connected directly to the sunward interplanetary magnetic field in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Using data from the low‐altitude DMSP spacecraft, we compare polar rain electron intensities, energy spectra, and time variations at low altitudes with concurrent bidirectional electron properties measured by ISEE 3. The field‐aligned lobe electron phase space densities above ∼200 eV agree well (i.e., to within a factor of ∼2–3) with DMSP‐measured polar rain phase space densities near the polar cap. Furthermore, the spectral slopes above 200 eV also are similar at low and high altitudes. Below 100–200 eV there is a thermal electron population in the distant tail, probably arising from local entry of plasma through the distant magnetopause, which is not present at DMSP altitudes. These data suggest 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.