Plasma pitch angle distributions near the substorm injection front

ATS-6 spacecraft hot plasma instrument data obtained during January, 1980 is presented, which provides electron and ion pitch distributions in the vicinity of an earthward-propagating, substorm-associated abrupt plasma change near synchronous orbit. Evidence is found of symmetric atmospheric source cones for few 100-eV electrons after front passage, supporting both (1) concept of atmospheric electron degradation of the hot, high-altitude plasma, and (2) the proposal that the injection front is a moving, precipitation-flow boundary between the hot plasma and the cooler plasma that has become spectrally degraded via interaction with the atmosphere. The enhanced hot plasma electron intensities appearing in association with front passage exhibit a modest, field-aligned anisotropy with minima at pitch angles characteristic of symmetric loss cones, consistent with mirror compression of the electrons on inward-collapsing field lines.