1Space Sciences Laboratory, University of California, Berkeley
2Naval Research Laboratory, Washington, DC
Electron observations by the WIND plasma instruments in the near-Earth plasma sheet (at a radial distance of about 10 Earth radii) during a substorm expansion and recovery reveal the presence of counter-streaming electron beams. The beams, which appear shortly after large fluctuations in the magnetic field, are centered at about 1 keV and are confined to pitch angles less than about 10 degrees. These beams appear to be unstable and rapidly decay resulting in bi-directional field-aligned electron distributions. The resulting distributions contain two components: a thermal, relatively isotropic plasma sheet component, and a lower energy, more strongly field-aligned beam remnant. The bi-directional field-aligned distributions are observed for more than one hour. Simultaneous FAST plasma measurements near the magnetic footprint of WIND in the auroral region show a similar two component electron spectrum. The source of the field-aligned beams is unknown, but based on the narrowness of the beams in the plasma sheet, we contend that the source is at low altitude and that the source mechanism is related to the auroral acceleration processes.
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