Energetic Electrons as a Field Line Topology Tracer in the Cusp Region: Cluster Rapid Observations

Energetic electron observations should be able to clearly delineate regions of open and closed magnetic field lines in the high latitude and thus contributed crucially to understanding and resolving an internal debate going on between groups measuring only the lower energy (<30 keV) plasma. Energetic electrons are unique to fully assess magnetic-field-line topology and thus distinguish between competing high latitude boundary / cusp dynamic models. The energetic electrons in the high latitude region (including cusp) has been examined in detail by using Cluster/RAPID data set for four consecutive high latitude/ cusp crossings with energies of about 20-400 keV between 16 March to 19 March 2001. The stable flux of energetic electrons are observed mainly in the time interval when the IMF had a predominate positive Bz component and were associated with lower plasma density and without an obvious tailward plasma flow (< 30 keV). No electrons or only spike -like electron events has been observed during IMF negative Bz whereas the plasma density is high and associated with a clear tailward flow. No stable energetic electron flux has been observed during IMF southward indicates that the cusp has a general open field line geometry. High flux energetic particle have been observed in the region very frequently, although a number of possible sources have been proposed, the origin of these energetic particles is still on debate. No matter where those particles come from, a mechanism is needed to maintain them in the cusp. The energetic electron observation provide new clues to understand the dynamic cusp process.