Escape of heated ions upstream of quasi-parallel shocks

We investigate the free escape of ions from the post shock plasma into the region upstream of a fast collisionless shock. We find that the accessibility of downstream ions to the upstream region depends strongly on the upstream magnetic field shock normal angle as well as the upstream plasma parameters. Post shock ions escape upstream in significant numbers for shock normal angles below about 450, in rough accord with the observed transition between quasi-perpendicular and quasi-parallel shock structure. We also calculate upstream ion distribution functions that resemble those of the observed ''intermediate'' ions and ion beams