Energetic (>0.2 MeV) electron bursts in the deep geomagnetic tail observed by the Goddard Space Flight Center experiment on ISEE 3: Association with geomagnetic substorms

Abstract

Brief (< 1‐hour duration) 0.2‐ to 2‐MeV electron bursts were detected in the geomagnetic tail out to 238 RE from Earth by the Goddard Space Flight Center medium energy cosmic ray experiment on ISEE 3 in 1982–1983. We examine their association with geomagnetic substorms and illustrate several examples in conjunction with simultaneous ISEE 3 plasma, magnetic field, and low‐energy (35–56 keV) ion data. The majority (>80%) of electron bursts in the plasma sheet and plasma sheet boundary layer (PSBL) were associated with AL intensifications and/or particle injections observed at geosynchronous orbit, indicative of a relationship with substorm onsets. Bursts were detected following less than 50% of apparent substorms, presumably because a burst was detected only if the spacecraft was located within the region of field lines reconnected at the substorm neutral line. In the deep tail (∼ 200 RE from Earth), peak energetic electron intensities were typically observed near the outer edge of the PSBL closely following substorm onset. This finding suggests that the electrons were accelerated on newly reconnected field lines and lost rapidly from the tail. Unlike low‐energy (tens of keV) ions, energetic electrons (at least with intensities above the detector background) were not a general feature of the PSBL or the plasma sheet outside times of enhanced substorm activity. Most plasmoids (∼80%) in the deep tail were not associated with enhanced energetic electron fluxes, a suggestion that electrons accelerated at the substorm neutral line during disconnection of the plasmoid may leak out from the plasmoid. This possibility is consistent with flux‐rope‐like magnetic structures in plasmoids which connect to open field lines at the flanks of the tail. Electron anisotropies can be examined during the more intense bursts. These show either weak flows consistent with trapping on closed field lines connected to the Earth or stronger, tailward flows which suggest flow away from an acceleration region earthward of the spacecraft. Weak flows were observed in the plasma sheet within ∼90 RE from Earth, consistent with previous ISEE 3 studies suggesting that this is the typical location of the distant neutral line. We confirm earlier observations that electron bursts in the magnetosheath are found predominantly close to the magnetopause, are associated with low‐energy ion enhancements, and show strong tailward flows. Since most of these bursts were detected within ∼90 RE of Earth, the region where intense plasma sheet electron bursts were also observed, this finding suggests that the plasma sheet may be source of the magnetosheath bursts.