Electron Acceleration by ICME-driven Shocks at 1 au

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 JD/JA > 1.5 across the shock, respectively at 0.43, 1.95, and 40 keV, where JD and JA are the electron flux in the shock's downstream and the preceding ambient solar wind. For significantly shocked suprathermal electrons, the differential flux JD positively correlates most with the magnetosonic Mach number Ms, while the flux enhancement JD/JA positively correlates most with the magnetic compression ratio rB, among the shock parameters. Both JD and JA generally fit well to a double-power-law spectrum at ∼0.4-100 keV, J ∝ E−β, with an index of β1 ∼ 2-6 below a break energy of Ebr (which is typically ∼2 keV) and an index of β2 ∼ 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, JD/JA 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.