Relativistic surfing acceleration of ions at oblique shocks

It was shown in [M.A. Lee, V.D. Shapiro, R.Z. Sagdeev, J. Geophys. Res. 101 (1996) 4777] that one of the limits to the shock surfing acceleration of ions at quasiperpendicular shocks is because of the obliqueness of the shock wave. In this Letter, a critical obliqueness is found for the relativistic shock wave, below which the energy gain of the particle due to the shock surfing acceleration is not limited by the obliqueness of the shock wave.