Post-onset development of the auroral absorption substorm in relation to magnetospheric electrons

In a previous paper we described the behaviour of the energetic electron precipitation at the onset of an auroral absorption substorm, on the basis of the dynamics of visible aurora and radio absorption in relation to electron measurements by the satellite GEOS-2. We now report an extension ofthat study to the post-onset activity of the same substorm. Clear differences emerge when comparing both the direction of propagation and the temporal development of the electron spectrum during the subsequent activity, with those at the onset. The velocities of the absorbing regions in the ionosphere are observed to have equatorward and eastward components, as opposed to the westward propagation at the onset. The maximum intensity of radio absorption during the substorm main phase was associated with more intense fluxes of high energy electrons than at the onset, but with the superposition of a component softening with time, in direct contrast to the rapid hardening seen at the onset. The softening spectrum during the time of maximum absorption is interpreted as the arrival of a gradient-curvature drifting population, and at this time the flux increases at small pitch angles were confined to the range 45-180 keV, and the isotropy of the electron pitch angle distribution was maximum.