Plasma and field signatures of poleward propagating auroral precipitation observed at the foot of the Geos 2 field line

Abstract

The 6300‐Å auroral emission intensity at Kilpisjärvi, Finland, was monitored during the 1978–1979 winter, and comparisons made with data from the geostationary Geos 2 spacecraft located on the same magnetic field line. Optical 6300‐Å events with a rise time of a few minutes and a decay time of about 1 hour were regularly observed, and four events from the night of December 18, 1978, were analyzed in detail. All‐sky camera data showed that the events arose from poleward propagating auroras associated with auroral breakup. The Geos 2 plasma and field signatures for these events were remarkably well defined and consistent. About 1 hour prior to an event the tailward component of the magnetic field at Geos 2 begins to grow; it relaxes suddenly to a dipolar configuration at the event onset. At the same time there is a short‐lived electric field enhancement up to 15 mV/m and a burst of ULF waves (a short irregular pulsation event). There is a dropout in E <500 eV electron intensity, a sudden rise in E >20‐keV intensity, and an onset of VLF activity, all of which persists for roughly 1 hour, in parallel with the decaying 6300‐Å emission. The observed electric field values are comparable to the inductive electric field expected from the observed magnetic field changes and appear to play a crucial role in the processes. The energy release associated with the 6300‐Å emission is comparable to the energy stored in the E <500‐eV electrons prior to the onset. These events appear to correspond to injection events, occurring at the onset of the locally detected expansion phase.