The velocity distributions of cometary protons picked up by the solar wind

Abstract

Velocity space distributions of picked up cometary protons were measured by the ion mass spectrometer on the Giotto spacecraft upstream of the Halley bow shock. Large pitch angle anisotropies were observed at all distances >1.2×106 km from the comet. As expected, pitch angle diffusion was much more rapid than energy diffusion. When the field was quasi‐parallel to the solar wind velocity vector, it was possible to discern the effect of pitch angle scattering by sunward propagating, field‐aligned hydromagnetic waves, but there is evidence for other scattering modes as well. For quasi‐perpendicular geometries, the pitch angle distribution was very asymmetric with phase space density peaks near pitch angles of 180°. It is suggested that the asymmetric pitch angle distribution may be caused by global rather than local wave‐particle interactions. Just outside the shock, the pitch angle distribution was nearly isotropic and the radius of the pickup shell increased significantly.