Structure of the Halley bow shock.

Abstract The similarities between cosmic ray mediated shocks and the bow shock of comet Halley are discussed briefly and a two-fluid model is presented. Since the shock is weak, we use a perturbation scheme to study the shock structure. We show that the shock is smoothed by the pick-up ions and that the length scale of the cometary ion foot is determined by the diffusive length scale of the cometary ions. Our results suggest that the spatial diffusion coefficient of the cometary ions is close to its “Bohm” limit in the vicinity of the shock. In addition, we suggest that small irregularities on the smoothed shock may be explained by means of a cometary ion driven instability.