Variations in ionospheric currents and electric fields in association with absorption spikes during the substorm expansion phase

Abstract

Small‐scale auroral absorption events (‘spikes’) have been associated with the onset of an auroral substorm. In this paper we present detailed observations of ionospheric currents, electric fields, and aurora at the time of two spikes observed on March 1, 1977. The distribution of electric fields in the E layer around the time of occurrence of such absorption events has been investigated using the Stare auroral radar system. We find that the poleward motion of a substorm‐activated arc cannot be explained by E × B drift in the ionosphere. We furthermore find a predominantly southward electric field equatorward of the active arc, while it can be either northward or southward pointing poleward of the arc. Thus a poleward expanding arc leaves a westward current in its wake. We also detect an increase in the westward field component south of the arc. The only region where the westward field dominates is within an auroral structure which moves westward near the beginning of one of the substorms investigated. An example is shown of the reduction of the electric field over a large area including that of the riometer site, lasting for a few minutes shortly after the occurrence of an absorption spike.