Plasma pitch angle distributions near the substorm injection front

Abstract

Earthward propagation of substorm‐associated, abrupt plasma changes has been observed near synchronous orbit (Moore et al., 1981). We report data obtained from the University of New Hampshire hot plasma instrument on the ATS‐6 spacecraft during January 1980 that provide electron and ion pitch angle distributions in the vicinity of such an injection front. Evidence is found of symmetric atmospheric source cones for few 100‐eV electrons after front passage. This supports the concept of atmospheric electron degradation of the hot high altitude plasma, and the proposal that the injection front is a moving precipitation‐flow boundary between the hot plasma and the cooler plasma, which has become spectrally degraded via interaction with the atmosphere. In contrast, ion source cone distributions are more sporadic and not clearly associated with the injection front passage. The enhanced hot plasma electron intensities that appear in association with front passage exhibit a modest field‐aligned anisotropy with a minimum at 90° pitch angle and secondary minima at small and large pitch angles characteristic of symmetric loss cones. This is consistent with mirror compression of these electrons on inward‐collapsing field lines. Ions of comparable energies do not exhibit the field alignment, probably because their bounce periods are comparable to or longer than the time scale of collapse so that their second invariants are not conserved.