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 in teraction of the solar wind with non-magnetic planets and comets. We calcul ate trajectories of individual contaminant particles for different orientat ions of the magnetic field. Density profiles are derived analytically for t he 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 contam inant particles are accelerated by the convective electric field of the sol ar wind which is produced by their relative motion. The particles have cycl oidal 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 giv es rise to filamentary coherent structures that extend throughout the veloc ity shear. We discuss the application of these results to understand the fo rmation of plasma tail rays in comets.