M. Scholer Max-Planck-Institut f. extraterr. Physik, Garching, Germany
While a Fermi type mechanism for the acceleration of ions at collisionless shocks in astrophysical settings is widely accepted, the injection mechanism, i.e., how a certain part of the ions are extracted from the thermal population and are injected into the acceleration process, remains a subject of ongoing discussion. We argue that no immediate insight can come from either obseervations or analytic theory, but that we have to rely on self-consistent kinetic simulations taking into account the collsionless shock structure. Observations have the drawback that the upstream distribution is already processed, i.e., it is not possible to unfold the energetic particle distribution into those particles freshly injected and those already accelerated by a Fermi mechanism. Analytic theory, on the other hand, has to make assumptions about the injection process, usually that those heated downstream ions escape upstream which have an upstream speed exceeding the shock velocity. We report on kinetic simulations of quasi-parallel shocks which show that those ions which partake in a Fermi type acceleration process leave the shock already energized after their first shock encounter with energies up to several times the shock ram energy. Injection at quasi-perpendicular shocks has been connected with the process of cross-field diffusion. We have determined that coefficient for a setup resembling the situation in a quasi-perpendicular shock as closely as possible. This diffusion coefficient seems to be too low in order to lead to suffucient injection. However, since after their first encounter the ions leave the shock already highly energized, the injection process working at quasi-parallel shocks can also work at rather oblique shocks. This considerably eases the injection problem at quasi-perpendicular shocks. In contrast to the solar wind thermal population there is no problem to inject and accelerate pickup ions at both, quasi-parallel and quasi-perpendiclar shocks, since these ions constitute alrady a suprathermal particle population.