Energetic Electrons in the Magnetosheath and Upstream of the Bow Shock

Abstract

The characteristics and transport of >200 keV electrons upstream of Earth's bow shock and in the outer magnetosheath are studied using nearly 4 years of data from the California Institute of Technology Electron/Isotope Spectrometer aboard IMP 8. In the upstream region, elevated electron intensities ranging from near the background level ∼0.1 (cm^² s sr)^(−1) up to ∼100 (cm^(²) s sr)^(−1) are observed about 2% of the time and typically persist for less than 5 min. These burst electrons stream intensely (ξ ∼ 0.5 to 1.5) sunward along the interplanetary magnetic field line. With the aid of a new coordinate system, it is demonstrated that the upstream bursts are most frequently seen on interplanetary field lines that trace to the inner magnetosheath, and that the total energy transported sunward by >200 keV upstream electrons averages ∼1.6 × 10^(14) ergs/s, a figure comparable to the tailward energy flow observed in the permanent layer of energetic electrons adjacent to the magnetopause. Bursts of energetic electrons in the outer magnetosheath have time scales similar to those observed upstream, but they are typically ∼10 times more intense and much less anisotropic (ξ ≲ 0.4). The net flow of these electrons is toward the shock and very nearly equals the net flow away from the shock in the adjacent upstream region. The magnetosheath data suggest that energetic electron transport in this region may be described by a leaky box model with a scattering mean free path ∼0.6 R_E. It is concluded that the energetic (>200 keV) component of upstream electrons originates downstream of the bow shock. Energetic electrons in the upstream region, in the outer magnetosheath, and in the magnetopause electron layer appear to be closely related and probably have a common origin in the inner magnetosheath or possibly the magnetosphere.

Files

Additional details