Quasi‐linear pitch angle and energy diffusion of pickup ions near comet Halley

The process of ion pickup in the environment of Halley's comet is studied in order to see if velocity diffusion driven by the observed level of turbulence can explain the observed development of the implanted ion distribution. The theoretical description used is based on a quasi‐linear approach and considers the implantation and transport of cometary ions along solar wind flow lines. To make such a study requires some way of extrapolating our measurements on the Giotto trajectory into the upstream region; models for mass loading and turbulence are used. A simplified kinetic equation describing the source, convection, and quasi‐linear velocity diffusion of the heavy cometary ions is solved numerically along flow lines parallel to the Sun‐comet line. Full two‐dimensional (pitch angle and velocity) distributions are obtained at positions along the Giotto trajectory, which are compared with measurements. This study finds that quasi‐linear theory, with our empirical model for the observed turbulence level, produces the right order of pitch angle diffusion.