Spectral characteristics of precipitating electrons associated with visible aurora in the premidnight oval during periods of substorm activity

Three late evening passes of the DMSP/F2 satellite over the northern auroral zone have been studied using simultaneous visible emissions and measurements from a zenith-looking, 16-channel, electron spectrometer. All three orbits occurred during periods of substorm activity; the second and third passes were consecutive. The auroral oval is conveniently divided into three latitudinal segments: a most pole-ward bright arc: a region of structured inverted-V precipitation; and the diffuse auroral region. The most poleward bright arc is characterized by directional fluxes occasionally in excess of 1010 (cm2s sr)-1 carried by two populations with temperatures of approx.100 eV and approx.600 eV. These fluxes, which apparently continue well into substorm recovery, are not easily reconciled with a simple model for pressure balance in the magnetotail. Presumably they reflect not well understood dynamic processes in the distant magnetotail. In the second regions precipitating electrons had average energies in excess of 1 keV and had spectral shapes that were either quasi-thermal or monoenergetic peaks superimposed on a secondary-electron background. Beams of low-energy (<1 keV) electrons frequently found in this region are consistent with localized field-aligned potential drops flickering on and off at altitudes not much greater than that of the satellite. In the diffuse auroral region the average energy of electrons from the central plasma sheet ranged up to 5 keV. In isolated patches within the diffuse auroral region and near its equatorward boundary, electrons with energies < or =100 eV and fluxes exceeding 1010 (cm2 sr keV)-1 were found in all three cases. We interpret these electrons as part of an originally cold plasma that became detached from the plasmasphere due to time-varying convective electric fields. The electrons were subsequently heated by the Landau damping of electromagnetic ion-cyclotron waves