Suprathermal electrons at Earth's bow shock

Abstract

A hot, suprathermal population of electrons is often present near the Earth's bow shock. The overall morphology of this hot population as observed with the Los Alamos/Garching fast plasma experiments on ISEE 1 and 2 in the energy range from 1 to 20 keV is much as described previously by others working primarily with more energetic particle measurements. In particular, at energies below 20 keV the flux of suprathermal electrons is most intense immediately downstream from the shock and decreases in intensity with increasing penetration into the magnetosheath. Upstream fluxes of electrons in this energy range are generally considerably weaker than are the downstream fluxes. The major new results from our ISEE measurements are as follows: (1) The suprathermal electrons are commonly found downstream from perpendicular and quasi‐perpendicular portions of the shock but not downstream from quasi‐parallel portions. (2) The suprathermal electron spectrum extends smoothly out of the shocked solar wind spectrum generally as a power law in energy with an exponent in the range from −3 to −4. (3) Angular distributions for the suprathermal electrons are generally isotropic immediately downstream of the shock ramp, but an anisotropy perpendicular to the magnetic field usually develops with increasing penetration into the magnetosheath. (4) Suprathermal electrons are often first observed within the shock layer itself as a field‐aligned beam escaping upstream. We interpret these observations in terms of a qualitative model wherein the suprathermal electrons are accelerated out of the solar wind thermal and halo populations as the solar wind convects across perpendicular portions of the shock. Subsequently, the suprathermal electrons leak back upstream along the magnetic field to form the backstreaming fluxes of energetic electrons commonly observed in the Earth's foreshock region. Although the mechanism of acceleration is presently uncertain, we believe it is unlikely that these electrons are accelerated by magnetic mirroring of upstream electrons at the shock or that they result from a simple adiabatic mapping of electron distributions from upstream to downstream.