Cosmic ray driven instabilities in a plasma flow upstream of a supernova shock

Abstract

Forward shocks in supernova remnants are known as efficient cosmic ray (CR) accelerators. They may convert a sizeable fraction of the shock ram pressure into relativistic particles with highly non-equilibrium distribution functions. We discuss here a specific unstable mode driven by the CR-current and CR-pressure gradients which is producing the magnetic fluctuations in a cold plasma of the shock upstream. The growth rates of the new CR-driven mode propagating obliquely to the mean magnetic field are presented in the short-wavelength limit and compared to the known Bell’s CR-current and Drury-Dorfi-Falle acoustic instabilities.