The physics of particle acceleration by collisionless shocks

Using analytic theory, test-particle simulations, and self-consistent hybrid simulations, we show that quasi-perpendicular shocks - those which propagate nearly perpendicular to the upstream magnetic field - accelerate particles directly out of the incident thermal population to energies much higher than the upstream ram energy of the plasma. It has already been established that quasi-parallel shocks - those which propagate nearly in the same direction as the upstream magnetic field - efficiently accelerate particles directly out of the incident thermal population; however, this has not yet been established for quasi-perpendicular shocks. Our results can be understood within the framework of the diffusive shock acceleration theory. We find that the accelerated-particle spectrum obtained from a more-general self-consistent hybrid plasma simulation are quantitatively consistent with a less-sophisticated test-particle simulation. The implications of this are discussed.