Energetic Particles in the Cusp: A Cluster/RAPID View

Abstract

Energetic particles have been persistently observed in the exterior cusp by different satellite missions such as POLAR, Cluster-II, Viking, ISEE etc. Yet the source and the acceleration mechanism of these particles have remained unclear. In this paper I review our studies of energetic particles in the cusp and the nearby high-latitude region of closed magnetospheric field lines (HLPS, high-latitude dayside plasma sheet) using the data obtained by the RAPID instrument onboard the Cluster-II satellites. We conducted a large scale statistical study to examine the dependence of the energetic particle fluxes in the cusp and HLPS on solar wind/IMF conditions as well as on geomagnetic activity. The study showed that energetic ion fluxes in the HLPS correlate strongly with substorm activity and electron fluxes with solar wind speed and geomagnetic activity. In the exterior cusp a clear correlation between lower energy ions (E 75 keV) correlated with substorm activity. Our case studies have shown that when IMF By dominates reconnection can take place near the cusp and release energetic particles from closed field lines to the cusp. Coupled with these detailed observations the statistical results imply that the energetic particles in the HLPS and the cusp originate in the near-Earth magnetotail from where they can drift to the HLPS region. From the HLPS the higher energy particles diffuse more or less directly into the cusp while the lower energy particles are released into the cusp by reconnection. These observations provide a consistent explanation for the cusp energetic particles without a need for significant local acceleration of shocked solar wind plasma to MeV energies. While some energy transfer from the electromagnetic waves to plasma particles is known to occur in the cusp it cannot explain the observations discussed here.