The proton ring current and its interaction with the plasmapause: Storm recovery phase

Using data from the Explorer 45 satellite, we have studied the interaction of the proton ring current with the plasmapause during the December 17, 1971, geomagnetic storm. This study concentrates on the storm recovery phase. The observed spatial, pitch angle, and energy distributions are shown for the near-midnight proton ring current for prestorm quiet, main phase, and recovery phase. Initial measured distribution functions f(υ⊥, υ∥) are also shown. During recovery phase at altitudes just above the plasmapause, ring current protons exhibit a pitch angle distribution that is nearly isotropic except for an empty loss cone. As the plasmapause is approached, this distribution switches rapidly to one that is peaked near α=π/2. We conclude that during recovery phase there exists a region just above the plasmapause in which the hot ring current plasma is stably trapped with negligible losses due to pitch angle scattering. As the plasmapause region is approached, moderate pitch angle diffusion is initiated. The spatial, energy, and time dependence of this process, its analysis via the resonant energy equation, and a comparison with an in situ estimate of the cold plasma density strongly indicate that scattering by ion cyclotron waves is the cause of the pitch angle diffusion. We thus further conclude that amplification of ion cyclotron waves by the interaction of the hot ring current plasma with the cold plasmaspheric plasma in the region of the plasmapause is an important loss process for the ring current particles during storm recovery phase. The energy range over which this process occurs is from ∼1 keV to several hundred keV. Low-altitude observations of low-energy protons through the local loss cone are consistent with and included in this picture of recovery phase processes.