Electron acceleration during tail collapse

Calculations are made to determine what changes are expected in auroral electron energy spectra and pitch angle distributions, as seen at low altitudes, when the tail collapses from an extended to a dipolar configuration. The calculations begin with an isotropic Maxwell-Boltzmann distribution and assume that electrons move toward the earth while the first two adiabatic invariants are conserved. A model for plasma and field line motion is chosen to approximate motions that have been observed during substorms. The calculated energy changes do not, however, depend sensitively upon the model and so should apply to almost any rapid inward plasma motion. The energy distribution remains Maxwellian, and both the energy of an individual electron and the Maxwell-Boltzmann characteristic energy increase by the same ratio as electrons move to lower field lines. There is a general tendency for electron fluxes to become slightly peaked at 90° if the first two invariants are conserved during acceleration. A brief comparison with data from rockets and low-altitude satellites suggests that electrons may often be accelerated by this mechanism. Some experimentally observed spectra, however, are clearly not consistent with the model discussed here and require the presence of a separate acceleration mechanism.