Electron acceleration by Earth’s bow shock: Wind observations

Abstract:

We present a comprehensive study of in situ electron acceleration during 140 crossings of Earth’s bow shock with good suprathermal electron observations by Wind 3DP instrument from 1995 through 2004. Among the selected 124 quasi-perpendicular (16 quasi-parallel) shock cases, ~91% (~44%), ~90% (~56%) and ~25% (~25%) show significant electron flux enhancements of J_D/J_A>1.5 across the shock, respectively at 0.43, 1.95, and 40 keV, where J_Dand J_Aare the electron flux in the shock’s downstream and the ambient solar wind. For significantly shocked suprathermal electrons, the downstream flux J_Das well as the flux enhancement J_D/J_Apositively correlate most with the magnetic compression ratio r_B, among the shock parameters. The downstream suprathermal electrons generally have three forms of spectra: a double power-law spectrum bending up at ~2 keV, a triple power-law spectrum bending up at ~2 keV then bending down at energies below 20 keV, and a double power-law spectrum bending down at energies between 2-20 keV. The first two forms of spectra generally retain the upward-bending shape of the ambient electron spectra, while the third spectral form appears to be unrelated with the ambient spectra. The downstream spectral index at ~0.5-2 keV (the flux enhancement J_D/J_Aat ~0.5-6 keV) observed in the triple-power-law cases appears to be smaller (larger) than that observed in the upward-bending double-power-law cases and larger (smaller) than that observed in the downward-bending double-power-law cases, indicating that the triple power-law spectrum could be a transition form from the upward-bending to the downward-bending power law. The evolution of these three forms of electron spectra suggest that the stochastic shock drift acceleration may play a role in the electron acceleration at Earth’s bow shock.