The structure of mass-loading shocks. 1: Comets

A new multifluid model to describe the solar wind-cometary ion plasma in the outer cometary coma is derived. This model is distinguished from previous multifluid models in that wave-particle effects are included explicitly. By considering hydrodynamic timescales, self-consistent dissipative terms (which correspond to the spatial diffusion of cometary ions) are derived, thereby allowing the structure of a cometary bow shock to be resolved. In the hypersonic limit, our diffusive multifluid model reveals an attractive connection to the one-fluid model formulated by Biermann et al. (1967), a connection which serves as an important guide when investigating our more complicated nonhypersonic diffusive model. The detailed model consists of cometary ions diffusing in a warm solar wind which is heated by the dissipation of Alfvenic turbulence (in situ and generated by the initial ring beam distribution). The quasi-parallel cometary shock is found to be at least an order of magnitude thicker than a quasi-perpendicular cometary shock. It is furthermore found that for undisturbed solar wind (sonic) Mach numbers in excess of approximately 6, the cometary bow shock (for both perpendicular and parallel cases) is smoothed completely by the cometary ions and no 'proton' or thermal subshock is necessary. Finally, a detailed comparison of the model with the plasma observations made at Halley by Giotto is undertaken. It is found that the observed shock structure, thickness, location, and plasma parameters compare well with the predictions of the theoretical model, particularly in the case of the quasi-parallel shock.