Turbulence transmission in parallel relativistic shocks using ray tracing

¹ Tuorla Observatory, Väisälä Institute for Space Physics and Astronomy, Väisäläntie 20, 21500 Piikkiö, Finland e-mail: joni.tammi@iki.fi
² 
UCD School of Mathematical Sciences, University College Dublin, Belfield, Dublin 4, Ireland

³ 
Department of Physical Sciences, PO Box 64, 00014 University of Helsinki, Finland

Received: 10 January 2006
Accepted: 6 September 2006

Abstract

We apply a semi-classical approach of handling waves as quasiparticle gas in a slowly varying flow – analogous to ray tracing – to calculate the Alfvén wave transmission parameters, the resulting cross-helicity of the waves and the scattering-centre compression ratio, for cases where the shock thickness is large enough for the turbulent waves in the plasma to see the transition of the background flow parameters as smooth and slowly varying. For nonrelativistic shocks the wave transmission produces similar effects on the downstream turbulence and the scattering-centre compression ratio as does the transmission through a step shock: the downstream Alfvén waves propagate predominantly towards the shock in the local plasma frame and, thus, the scattering-centre compression ratio is larger than the gas compression ratio. For thick relativistic shocks, however, we find qualitative differences with respect to the step-shock case: for low-Alfvénic-Mach-number shocks the downstream waves propagate predominantly away from the shock, and the scattering-centre compression ratio is lower than that of the gas. Thus, when taken into account, the Alfvén wave transmission can decrease the efficiency of the first-order Fermi acceleration in a thick relativistic shock.