Electron Acceleration by ICME-driven Shocks at 1 au

Abstract

Abstract We present a comprehensive study of in situ electron acceleration during 74 shocks driven by interplanetary coronal mass ejections (ICMEs) with good suprathermal electron observations by the Wind 3DP instrument at 1 au from 1995 through 2014. Among the selected 59 quasi-perpendicular (15 quasi-parallel) shock cases, ∼86% (∼60%), ∼62% (∼36%), and ∼17% (∼7%) 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 D and J A are the electron flux in the shock’s downstream and the preceding ambient solar wind. For significantly shocked suprathermal electrons, the differential flux J D positively correlates most with the magnetosonic Mach number M s , while the flux enhancement J D /J A positively correlates most with the magnetic compression ratio r B , among the shock parameters. Both J D and J A generally fit well to a double-power-law spectrum at ∼0.4–100 keV, J ∝ E −β , with an index of β 1 ∼ 2–6 below a break energy of E br (which is typically ∼2 keV) and an index of β 2 ∼ 2.0–3.2 at energies above. is similar to in all the shock cases, while is similar to (larger than) in ∼60% (∼40%) of the shock cases with significant electron enhancements. Furthermore, J D /J A mostly peaks in the directions perpendicular to the interplanetary magnetic field at ∼0.4–50 keV. These results suggest that both quasi-parallel and quasi-perpendicular shocks accelerate electrons in situ at 1 au mainly via shock drift acceleration, with an acceleration efficiency probably affected by the induced electric field at the shock surface.