Sounding rocket measurements of suprathermal ion acceleration
Transverse ion acceleration has been observed at rocket altitudes between 500 and 1000 km due to the injection of 100-200-eV argon plasma, auroral electron precipitation, and the injection of electromagnetic waves. Field-aligned currents necessary to neutralize the plasma injection payloads and those naturally occurring in the aurora could be responsible for the ions observed in the first two observations. Associated with the aurora, both bulk heating and tail heating are observed, sometimes simultaneously. In this case, either different masses are accelerated and/or different mechanisms are responsible. The bulk heating is closely correlated with the aurora structure while tail heating is not so well correlated. High-time-resolution rocket ion data have revealed that the transverse acceleration process is of very short duration ( approximately 100 ms) and occurs in a very limited volume (a few hundred kilometers along B and on the order of the ion gyrodiameter across B). Such impulse acceleration events are correlated with waves near the lower hybrid resonance. Wave injections of electromagnetic waves near the lower hybrid frequency result in the transverse acceleration of ambient ions.