Collisionless quasiperpendicular shocks with magnetoacoustic Mach numbers exceeding a
certain threshold are known to reflect a fraction of the upstream ion population. These
reflected ions drive instabilities which, in a magnetized plasma, can give rise to electron
acceleration. In the case of shocks associated with supernova remnants SNRs, electrons
energized in this way may provide a seed population for subsequent acceleration to highly
relativistic energies. If the plasma is weakly magnetized, in the sense that the electron …