High‐resolution energetic particle measurements at 6.6 RE 3. Low‐energy electron anisotropies and short‐term substorm predictions

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Abstract
Multiple detectors giving nearly complete 4π coverage of particle pitch angle distributions have provided high resolution measurements (in energy and time) of 30‐ to 300‐keV electrons. Data from a spacecraft (1976‐059A) in geostationary orbit show a remarkably consistent sequence of variations of the electron anisotropy before and during magnetospheric substorms. For periods typically 1–2 hours prior to the onset of substorms, electron distributions, peaked along the direction of the local magnetic field, are observed in the premidnight sector. These cigarlike anisotropies are accompanied by a local taillike magnetic field which may develop further during the event. At substorm onset an abrupt transition usually occurs from the cigar‐shaped distributions to pancake‐shaped distributions. This anisotropy sequence may be due to the buildup and subsequent release of stresses in the magnetotail; the cigar phase may also be due to associated processes at the dayside magnetopause causing a loss of 90° pitch angle particles. The present observations, based on ∼100 events, appear to provide a predictive tool for assessing the probability of occurrence of a substorm.