Injection and Acceleration of Thermal Ions at Collisionless Shocks:
Results of Kinetic Simulations
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 īprocessedī, 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.