THEMIS observations of compressional Pc5 pulsations in the dawn- and duskside magnetosphere

Abstract

We present case and statistical studies of compressional Pc5 pulsations observed by the THEMIS spacecraft in the dawn- and duskside magnetosphere from 2007 to 2011. A case study provides evidence for compressional Pc5 pulsations in the dawnside magnetosphere following the arrival of particles injected by a substorm and the subsequent growth of the drift mirror instability on February 27, 2011. The statistical study shows that the Pc5 pulsations occur in a weak or moderately disturbed magnetosphere but rarely develop during strong magnetic activity (i. e., for Dst index below −25 nT). The most striking characteristic of the compressional Pc5 pulsations is their localization in latitude and longitude over many consecutive days. Dawnside Pc5 pulsations occur under quieter conditions, last longer, and attain greater amplitudes than duskside Pc5 pulsations. We attribute compressional pulsations in both sectors to the drift mirror instability, with a warmer, more energized, and gradient-curvature drifting population of substorm-injected ions on the duskside and a colder ExB drifting population of plasma sheet ions on the dawnside.