Injection and acceleration of ions at collisionless shocks: Kinetic simulations

Abstract : Kinetic simulations of collisionless shocks have provided a wealth of information on injection and acceleration of thermal ions into a diffusive acceleration process. At quasi-parallel shocks upstream diffuse ions and the induced upstream turbulence are an integral part of the collisionless shock structure. Before injected into a diffusive acceleration process thermal ions are trapped near the shock and are accelerated to higher energies. The injection and acceleration process for thermal ions at quasi-perpendicular shocks depends on the possibility of these ions to recross the shock many times. A viable mechanism for injection is cross-field diffusion of the specularly reflected ions after they have crossed the shock into the downstream region. Determination of the cross-field diffusion coefficient in strong turbulence suggests that specularly reflected ions can recross the quasi-perpendicular shock and can get further accelerated. At more oblique shocks the same injection process as at quasi-parallel shocks can work: particles gain high enough velocities during their first shock encounter so that they can escape the shock along the magnetic field in the upstream direction. Because of the form of the pickup ion distribution in velocity space there seems to be no problem for accelerating these ions at either quasi-parallel, quasi-perpendicular, or perpendicular shocks.