Spatial distribution of pick-up ions in velocity shears I. The origin of cometary plasma rays

The purpose of this study is to examine the spatial distribution of pick-up ions in a region with a macroscopic velocity shear, as may occur in the interaction of the solar wind with non-magnetic planets and comets. We calculate trajectories of individual contaminant particles for different orientations of the magnetic field. Density profiles are derived analytically for the case in which the source of ions is infinite in extent, and numerically for the case in which the source is confined to a finite reg on. The contaminant particles are accelerated by the convective electric field of the solar wind which is produced by their relative motion. The particles have cycloidal trajectories in which their residence time is not uniform but varies across the velocity shear. This inhomogeneity in the velocity of the pickup ions leads to a greater particle density in the vicinity of the apexes of their cycloidal trajectories. As a result the distribution of particles gives rise to filamentary coherent structures that extend throughout the velocity shear. We discuss the application of these results to understand the formation of plasma tail rays in comets.