The Role of Plasma in Radiation Belt Loss.

Abstract:

The radiation belts are zones of relativistic electrons encircling the Earth. Their radial structure is controlled by the competition between source and loss processes. Most commonly, a two-belt structure prevails, though a more complicated three-belt structure - an inner belt plus two outer electron belts - have repeatedly been observed.

The plasma conditions that enable and enhance loss-facilitating wave activity in the inner magnetosphere are still under discussion. Relativistic electrons have been thought to more easily resonate with electromagnetic ion-cyclotron waves (EMIC) when the total plasma density is large (i.e., in the plasmasphere and plume). However, there is evidence that this interaction may be not as strong as thought, and that instead the field-aligned motion of lower energy ring current ions (up to a few 10's keV) may play a key role. Similarly, the exact influence of large heavy ion (O+) concentrations remains unsettled.

We use 2.5+ years of Van Allen Probes observations to study the region of plasmasphere-outer belt overlap (and its vicinity). By now, the Van Allen Probes provide a complete and very dense coverage of the complete magnetosphere inside geosynchronous orbit We focus our interest on understanding the plasma conditions that can favor EMIC wave growth. We investigate the temperature anisotropy A (modified by plasma β) of the warm/hot plasma, and contrast it with the location specifics of the plasmasphere (i.e., very high total density) and the occurrence of high O+ concentrations in the overlap regions with the radiation belt(s). We present both average conditions for all parameters during a variety of geomagnetic conditions, and highlight specific loss and overlap events in an effort to establish favorable plasma conditions for relativistic electron loss during those times.