Electron injection at high Mach number quasi-perpendicular shocks: Surfing and drift acceleration

The process of electron injection at high Mach number, collisionless, quasi-perpendicular shock waves is investigated by means of one-dimensional electromagnetic particle-in-cell simulations. We find that energetic electrons are generated in two steps: (1) electrons are accelerated nearly perpendicular to the local magnetic field by shock surfing acceleration at the leading edge of the shock transition region, and (2) these preaccelerated electrons are further accelerated by shock drift acceleration. As a result, energetic electrons are preferentially reflected back upstream. Shock surfing acceleration provides sufficient energy for the reflection. Therefore, it is important not only for the energization process itself, but also for triggering the secondary acceleration. We also present a theoretical model of the two-step acceleration mechanism, based on the simulation results, that can predict the injection efficiency for a subsequent diffusive shock acceleration process. We show that the injection efficiency obtained in the present model agrees well with the value obtained from Chandra X-ray observations of SN 1006. At typical supernova remnant shocks, energetic electrons injected by this mechanism can self-generate upstream Alfven waves, which scatter the energetic electrons themselves.