Energetic (>0.2 MeV) Electron Bursts in the Deep Geomagnetic Tail Observed by ISEE 3: Association with Substorms and Magnetotail Structures

Abstract

Brief 0.2-2 MeV electron bursts were detected in the plasma sheet, tail lobes and magnetosheath by the Goddard Space Flight Center Medium Energy Cosmic Ray Experiment during the ISEE 3 geotail mission in 1982-3. We summarize the main features of these data and their implications for the structure and dynamics of the geomagnetic tail. Sharp falls in the intensity and occurrence rate of plasma sheet events at ∼90 Re from Earth suggest a change from predominantly closed field lines Earthward of this location to predominantly open field lines at greater distances downtail. The electron anisotropies and associated plasma flows support this proposal which is also consistent with other ISEE 3 observations suggesting that the tail neutral line typically lies at around this distance. Electron bursts in the tail are closely association with substorm activity onsets as indicated by AL intensifications and particle injections at geosynchronous orbit. Signatures of plasmoid ejection down the tail may be observed following electron bursts. In the deep tail, energetic electron enhancements tend to be observed close to the time of substorm onset and are not generally present in the plasma sheet and plasma sheet boundary layer at other times. This suggests that the electrons are accelerated at the substorm neutral line rather than along the current sheet, and are lost rapidly from the tail along open field lines.