Fermi acceleration of auroral particles

A number of nighttime acceleration mechanisms proposed in the literature are reviewed and rejected for the role of producing the kev nighttime auroral-particle fluxes. Parallel electric fields are rejected for several reasons, but particularly because of the observed simultaneous precipitation of electrons and protons. Acceleration in the neutral sheet is inadequate for producing the particle energies, the observed field-aligned pitch-angle distribution at high latitudes, and the spectral hardening toward lower latitudes. Neutral point mechanisms, although often suggested in principle, have never been demonstrated satisfactorily in theory or in practice. Pitch-angle scattering from a trapped population produced by transverse adiabatic compression is also incapable of producing the field-aligned distribution. We therefore suggest that longitudinal or Fermi acceleration, which results from the known magnetospheric convection, is the main nighttime auroral acceleration mechanism. The argument is supported by data obtained with the soft-particle spectrometer on Isis 1. In particular, the rotational scanning provided by the spinning satellite shows the regular change from a field-aligned pitch-angle distribution at high latitudes to a trapped distribution at low latitudes, reflecting a change from Fermi acceleration on the distant taillike field lines to betatron acceleration on the more dipolar field lines closer to the earth. The operation of the Fermi mechanism requires multiple bounces between hemispheres, and therefore a loss cone must develop at both 0° and 180°; such distribution is shown by the Isis 1 data when the range of the pitch-angle scan is sufficiently wide. One other feature is evident in the data: a hardening of the spectrum at small pitch angles. This energy-dependent pitch-angle distribution has a characteristic signature on the energy-time spectrograms that is designated as a Λ structure; when both loss cones show, the characteristic signature is a topless Λ structure. Adiabatic compression thus plays a vital role in auroral and magnetospheric substorms; longitudinal or Fermi acceleration causes auroral-particle precipitation, whereas transverse or betatron acceleration is responsible for lower-latitude phenomena, such as the ring current.