Cluster encounter with an energetic electron beam during a substorm

Abstract

During a passage of the midtail plasma sheet, the Cluster 3 spacecraft made an unprecedented high-resolution measurement of a beam of electrons with energies up to 400 keV. The beam was only fully resolved by combining the energy range coverage of the Plasma Electron and Current Experiment and Research with Adaptive Particle Imaging Detector electron detectors and its pitch angle distribution evolved from antiparallel, through counterstreaming to parallel over a period of 20 s. Although energetic electron fluxes (similar to few hundred keV) are frequently observed in the plasma sheet, electron beams of this nature have rarely, if ever, been reported. The global conditions of the magnetosphere at this time were analyzed using multiple spacecraft, ground, and auroral observations, and are shown to be consistent with a near-Earth neutral line substorm scenario. The beam event is clearly associated with a substorm, and we present a discussion on the detailed analysis of the energy and time dependence of the beam in context with several current theories of particle acceleration, finding that the observations do no fit completely with any in a straightforward way.