6 Electron Injection at High Mach Number Quasi - Perpendicular Shocks : Surfing and Drift Acceleration

Electron injection process 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 through the following 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. (2) the preaccelerated electrons suffer shock drift acceleration because of their large energy and pitch angle. As a result, energetic electrons are preferentially reflected back to the upstream. Shock surfing acceleration is important not only for the energization process itself , but also for triggering the second acceleration process. We also present a theoretical model based on the two-step acceleration mechanism, which predicts the injection efficiency for subsequent diffusive shock acceleration process. We show that the injection efficiency obtained by the present model roughly agrees with that obtained by Chandra X-ray observations of SN 1006.