Observations of Electrostatic Shocks and Associated Plasma Instabilities by the S3-3 Satellite

The polar orbiting S3-3 satellite provides an opportunity to study plasma phenomena occuring on auroral magnetic field lines below 8000 km because the on-board instruments measure AC and DC electric fields and thermal particle densities, energetic ions and electrons, and field-aligned currents. Field-aligned currents above the threshold for excitation of ion cyclotron waves occur in association with ion cyclotron waves, electrostatic shocks and parallel electric fields, suggesting that thermal electron drifts supply the free energy to support these processes. Comparisons with computer simulations suggest that the currents excite plasma instabilities producing parallel electric fields via anomalous resistivity. An electrostatic shock may then develop, supported by ion beams and “runaway” electrons accelerated by the parallel electric field, and by the lower energy reflected particles produced by the turbulence. Extremely narrow band ion cyclotron waves have been identified in the broadband data, associated with both currents and ion beams, either of which could produce these waves. The parallel component of the electrostatic shock electric field has been measured simultaneously with the perpendicular component. The structure of electrostatic shock electric fields is shown to vary from cases with small parallel electric fields to cases with parallel fields larger than 100 mV/m. In virtually all cases, the perpendicular field magnitude exceeds the parallel field magnitude.