Observations of Ionospheric Electron Beams in the Plasma Sheet

Electrons streaming along the magnetic field direction are frequently observed in the plasma sheet of Earth's geomagnetic tail. The impact of these field-aligned electrons on the dynamics of the geomagnetic tail is however not well understood. Here we report the first detection of field-aligned electrons with fluxes increasing at similar to 1 keV forming a "cool" beam just prior to the dissipation of energy in the current sheet. These field-aligned beams at similar to 15 R-E in the plasma sheet are nearly identical to those commonly observed at auroral altitudes, suggesting the beams are auroral electrons accelerated upward by electric fields parallel (E-parallel to) to the geomagnetic field. The density of the beams relative to the ambient electron density is delta n(b)/n(e) similar to 5-13% and the current carried by the beams is similar to 10(-8)-10(-7) Am-2. These beams in high beta plasmas with large density and temperature gradients appear to satisfy the Bohm criteria to initiate current driven instabilities.