The acceleration and precipitation of nightside auroral particles

Abstract

Low-energy particle measurements by the Soft Particle Spectrometer on the ISIS-1 polar orbiting satellite reveal a number of clues as to the mechanism or mechanisms acting to produce auroral particles. Nightside particle precipitation patterns are presented in the format of the energy-time spectrogram, which showed differential spectral measurements over the range 10 ev to 10 kev for electrons and protons. Four of the passes are compared with auroral activity as recorded by all sky cameras. The nightside observations show that in the midnight sector electrons and protons are often precipitated simultaneously, and the spectral measurements generally indicate a gradual increase in the average particle energy with decreasing latitude in the auroral zone. In addition the rotational scanning provided by the spinning satellite shows a change in the angular distribution from the field aligned at high latitude to isotrophic at low latitudes. This reflects a change from longitudinal (Fermi) acceleration on the distant tail-like field lines to transverse (betatron) acceleration on the more dipolar field lines nearer the earth. The field aligned fluxes show an increase in spectral peak energy as the detector scans through small pitch angles, and this appears on the energy-time spectrogram as a spin-dependent A-structure. Operation of the Fermi process in the tail requires multiple bounces on closed field lines, and this is shown by the presence of a conjugate hemisphere loss cone, observed when the pitch angle scan is sufficiently wide. This spin dependent structure appears on the spectrogram as a topless A-structure. The particle precipitation patterns are interpreted as evidence for the acceleration of auroral particles principally by adiabatic compression in which the longitudinal acceleration becomes enhanced over transverse acceleration due to the extended nature of the magnetotail. Two other accelerations mechanisms seriously considered in the literature, parallel electric fields and neutral point ejection as well as pitch angle scattering are ruled out as primary auroral acceleration processes although each may be operative in a secondary capacity. Other experimental evidence as presently exists provides support for these assertions.