Particle acceleration at oblique shocks and discontinuities of the densityprofile

In the theory of diffusive acceleration at oblique shock fronts the questio n of the existence of a discontinuity of energetic particle density is cont entious. The resolution of this problem is interesting from a theoretical p oint of view, and potentially for the interpretation of observations of par ticle densities at heliospheric shocks and of high-resolution radio observa tions of the rims of supernova remnants. It can be shown analytically that an isotropic particle distribution at a shock front implies continuity of t he particle density - whether or not the shock is oblique. However, if the obliquity of the shock induces an anisotropy, a jump is permitted. Both sem i-analytic computations and Monte-Carlo simulations are used to show that, for interesting parameter ranges, a jump is indeed produced, with accelerat ed particles concentrated in a precursor ahead of the shock front.