  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 are further accelerated by shock drift acceleration. As a result,
energetic electrons are preferentially reflected back to the upstream. Shock
surfing acceleration provides sufficient energy required for the reflection.
Therefore, it is important not only for the energization process by itself, but
also for triggering the secondary acceleration process. We also present a
theoretical model of the two-step acceleration mechanism based on the
simulation results, which can predict the injection efficiency for subsequent
diffusive shock acceleration process. We show that the injection efficiency
obtained by the present model agrees well with the value obtained by Chandra
X-ray observations of SN 1006. At typical supernova remnant shocks, energetic
electrons injected by the present mechanism can self-generate upstream Alfven
waves, which scatter the energetic electrons themselves.
