ENERGETIC (GREATER-THAN-0.2 MEV) ELECTRON BURSTS OBSERVED BY ISEE-3 IN THE DEEP (LESS-THAN-240 RE) GEOMAGNETIC TAIL

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 approximately 240 RE from the Earth during the geotail mission in 1982-1983. The most n otable feature of these data is the presence of brief (< 1 -hour durat ion) electron bursts which exceed the instrument background of approxi mately 0.02 (s cm2 sr MeV)-1. These occur most frequently and have the highest intensities, within approximately 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 freque ntly on closed field lines connected to the Earth, based on their asso ciation with slow earthward or mixed plasma sheet flows and northward directed magnetic fields. The near-absence of such events beyond appro ximately 80 R(E) downtail provides evidence for predominantly open rat her than closed magnetic field structures in the plasma sheet beyond t his distance downtail. The observations are consistent with the presen ce of a distant neutral line in the tail plasma sheet at an average di stance of approximately 80 R(E) from the Earth. Approximately 30% of p lasma sheet electron bursts are associated with north then south turni ng magnetic field deviations and fast tailward plasma flows, which may be related to the expulsion of plasmoids following substorm onsets. A pproximately 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 wi th downtail distance. We show examples of plasma sheet and lobe electr on bursts that are clearly associated with substorms, indicating that temporal as well as spatial variations in the energetic electron inten sity do indeed occur in the geomagnetic tail. Finally, approximately 2 0% of the electron bursts are observed in the magnetosheath. The inten sity of magnetosheath bursts also falls off with downtail distance suc h that they are nearly absent beyond approximately 100 R(E) from the E arth.