Comet tail streamers in the solar wind

Abstract

Transverse plasma instabilities are suggested as the basis of the mechanism which couples the motions of the solar wind and cometary plasma. In a plasma with a non-isotropic distribution of particle velocities, collective interaction of the charged particles can lead to electromagnetic instabilities. In a shearing flow these plasma instabilities draw energy from that of the shear and ensure a short time of free flight for diffusing ions. We derive a coefficient of viscosity for a mixed CO +, H + and electron plasma based on this mechanism. Using in addition the related transport coefficients of thermal diffusion and conductivity we describe the flow of a cometary streamer in the supersonic solar wind. Viscous heating of the cometary plasma is shown to be important and is taken into account in the derivation of a similarity solution for the steady flow of a straight streamer. The length of a typical streamer is predicted to be about one hundred times its width, showing that the stability of comet rays against diffusion can be explained without invoking a magnetic field. The hydrodynamic stability and extreme straightness of the tail rays are due to the hypersonic nature of the flow. Finally, the variation with distance of the density and velocity of ions in the tail rays is deduced, and is compared with observations.