The Magnetospheric Cusp: Structure and Dynamics

Abstract

Understanding the polar cusps is essential for a thorough understanding of the entire physics of the magnetosphere, and of the dynamical interaction between the solar wind and any planetary magnetosphere. Energetic electrons are unique to fully assess magnetic field-line topology and thus should be able to clearly delineate regions of open and closed magnetic field lines in the high-latitude regions and contributed crucially to understanding and resolving an internal debate going on between groups measuring only the lower energy (< 20 keV) plasma. Energetic electrons with high and stable flux were observed in the high latitude boundary/cusp region when the Interplanetary Magnetic Field (IMF) had a predominate positive Bz component. With measurements at larger separations and more coordination of multiple satellite measurements for particular cusp crossings it will become more evident what the true nature of the cusp is and what roles the cusp plays. The boundary normal, velocity and timing analysis obtained by all four Cluster spacecraft indicates that the multiple cusp phenomena are most likely caused by the oscillation of a single northern cusp which was shifted back and forth. Cusp oscillations with a period of ∼20min are observed by Cluster in the high-latitude region, whilst the cold-dense plasma with fluctuations (20-min period) are observed in the dusk-side of the tail plasma sheet by Geotail. This is consistent with the idea that the high latitude reconnection during northward IMF is the responsible mechanism for the formation of the cold-dense plasma sheet.