The Electron Acceleration by ICME-driven Shocks at 1 AU During 1995-2014

We present a comprehensive study of the in-situ electron acceleration during 74 ICME-driven shocks observed by the WIND 3DP instrument at 1 AU from 1995 through 2014. We find that for both the quasi-perpendicular and quasi-parallel shocks, the shocked electron fluxes in the downstream, J_D, are positively correlated with the magnetosonic Mach number Ms, while the ratio of downstream over ambient fluxes, J_D/J_A, is positively correlated with the magnetic compression ratio r_B. And the downstream electron fluxes J_D (the ratio J_D/J_A) show positive correlation with the downstream proton fluxes J_D (the ratio J_D/J_A). Furthermore, the shocked electron differential fluxes at 0.4-100 keV in the downstream fit well to a double-power-law spectrum, J E^-β, with an index of β 2-6 at energies below a break at 2 keV and of β 1.6-3.2 at energies above. At energies below (above) the spectral break, the downstream electron spectral indices show inverse correlation (no clear correlation) with Ms and are similar to (similar to or larger than) the ambient spectral indices. The downstream spectral indices are mostly larger than the predication from first-order Fermi acceleration for a steady-state planar shock, and they appear not to correlate with both the whistler critical Mach number and the first magentosonic Mach number. On the other hand, we find that in 80% of shock cases, the average ratio J_D/J_A is the largest in the direction perpendicular to the interplanetary magnetic field, at all energies of 0.4-100 keV. These results suggest that the shock electron acceleration at 1 AU may favor the shock drift acceleration. These interplanetary shocks could accelerate and isotropize solar wind strahl/halo electrons at energies <2 keV and contribute to the production of solar wind superhalo electrons at energies >2 keV, in interplanetary space.