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Abstract

It is known that transverse hydromagnetic waves propagating parallel to the mean magnetic field are unstable to anisotropies in the cosmic ray distribution. We quantify the extent to which conditions in the precursor environment of a supernova shock, such as thermal and ionisation properties, limit the growth of such waves. If the cosmic ray acceleration is efficient, we find that very large neutral fractions are required to stabilise the growth of the non-resonant mode. For weakly driven modes, ion-neutral damping can dominate over the instability at more modest ionisation fractions. In the case of a supernova shock interacting with a molecular clouds, such as in RX J1713.7-3946, with high density and low ionisation, the modes can be rapidly damped.