Energetic (>0.2 MeV) electron bursts observed by ISEE 3 in the deep (<240 RE) geomagnetic tail

Abstract

The Goddard Space Flight Center medium‐energy cosmic ray experiment on ISEE 3 made as yet unreported observations of 0.2‐2 MeV electrons and >1 MeV/amu ions in the deep geomagnetic tail out to ∼240 RE from the Earth during the geotail mission in 1982‐1983. The most notable feature of these data is the presence of brief (<1‐hour duration) electron bursts which exceed the instrument background of ∼0.02 (s cm2; sr MeV)−1. These occur most frequently and have the highest intensities, within ∼80 RE of the Earth and are relatively absent at greater distances downtail. Approximately 60% of these bursts are observed in the plasma sheet. These occur most frequently on closed field lines connected to the Earth, based on their association with slow earthward or mixed plasma sheet flows and northward directed magnetic fields. The near‐absence of such events beyond ∼80 RE downtail provides evidence for predominantly open rather than closed magnetic field structures in the plasma sheet beyond this distance downtail. The observations are consistent with the presence of a distant neutral line in the tail plasma sheet at an average distance of ∼80 RE from the Earth. Approximately 30% of plasma sheet electron bursts are associated with north then south turning magnetic field deviations and fast tailward plasma flows, which may be related to the expulsion of plasmoids following substorm onsets. Approximately 20% of all the electron bursts are seen in the tail lobes, probably associated with encounters with the plasma sheet boundary (separatrix) layer. The lobe events show no clear trend in intensity with downtail distance. We show examples of plasma sheet and lobe electron bursts that are clearly associated with substorms, indicating that temporal as well as spatial variations in the energetic electron intensity do indeed occur in the geomagnetic tail. Finally, ∼20% of the electron bursts are observed in the magnetosheath. The intensity of magnetosheath bursts also falls off with downtail distance such that they are nearly absent beyond ∼100 RE from the Earth.