title: Observations of ionospheric electron beams in the plasma sheet.
abstract: 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 ~1 keV forming a "cool" beam just prior to the dissipation of energy in the current sheet. These field-aligned beams at ~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 the geomagnetic field. The density of the beams relative to the ambient electron density is δn(b)/n(e)~5-13% and the current carried by the beams is ~10(-8)-10(-7) A m(-2). These beams in high β plasmas with large density and temperature gradients appear to satisfy the Bohm criteria to initiate current driven instabilities.
authors.affiliation: School of Earth and Space Sciences, Peking University, Beijing 100871, China.
authors.forename: H
authors.initials: H
authors.lastname: Zheng
authors.forename: S Y
authors.initials: SY
authors.lastname: Fu
authors.forename: Q G
authors.initials: QG
authors.lastname: Zong
authors.forename: Z Y
authors.initials: ZY
authors.lastname: Pu
authors.forename: Y F
authors.initials: YF
authors.lastname: Wang
authors.forename: G K
authors.initials: GK
authors.lastname: Parks