THEORETICAL CALCULATIONS OF ION-ACCELERATION IN THE VICINITY OF COMETGIACOBINI-ZINNER

Ionization of cometary neutral molecules produces ions which are picke d up by the solar wind. The cometary ion pickup process for comet Giac obini-Zinner is studied in two ways: (1) with a test particle method i n which trajectories are numerically calculated for several thousand i ons whose initial locations were chosen randomly with probability prop ortional to the neutral density and (2) with a quasi-linear diffusion model. The cometary ion distribution function was calculated with the test particle model at several locations upstream of the bow shock and for several types of magnetic fluctuations (or waves). These waves we re allowed to propagate in both directions along the magnetic field at the Alfven speed. Both pitch angle scattering and energy diffusion ar e evident in the derived ion distributions. The monochromatic waves re sult in less ion acceleration than turbulent fluctuations with about t he same amplitude. The calculated ion distribution functions are in re asonable agreement with the distributions measured in the vicinity of comet Giacobini-Zinner in 1985 by particle detectors on the ICE spacec raft when the ratio of power in sunward propagating Alfven waves to th e power in antisunward propagating waves is assumed to lie between abo ut 20% and 50%. However, the quasi-linear diffusion model results agre e best with the measured distribution functions when the power ratio i s only about 3%.