Cosmic ray acceleration at oblique shocks
  We show that the diffusion approximation breaks down for particle acceleration at oblique shocks with velocities typical of young supernova remnants. Higher order anisotropies flatten the spectral index at quasi-parallel shocks and steepen the spectral index at quasi-perpendicular shocks. We compare the theory with observed spectral indices. 
A.R. Bell, K.M. Schure and B. Reville
astro-ph.HE