ION THERMALIZATION IN THE INNER COMA OF A COMET

The evolution of the newly created, picked-up cometary ion velocity di stribution in the inner coma of comet Halley is studied. Calculations are carried out for subsolar distances ranging from just inside the bo w shock (almost-equal-to 350,000 km for comet P/Halley) to a cometocen tric distance of about 9400 km which is a few thousand kilometers outs ide of the diamagnetic cavity boundary (or contact surface). The effec ts of energy diffusion, Coulomb collisons, charge transfer collisions, dissociative recombination, and ion-neutral polarization interactions have been included in these numerical calculations. Our results indic ate that the ion distribution function retains its shell-like nature f or radial distances greater than about 15,000 km, although charge tran sfer collisions and wave-particle interactions tend to cool and thicke n the shell, respectively. Our results also indicate that collisional processes such as ion-neutral polarization and Coulomb collisions ther malize the ion distribution function for distances inside about 15,000 km. We found that almost complete thermalization of a low-energy core of the distribution occurs at a distance of 1-2 x 10(4) km. The exist ence of a high-energy tail of the ion distribution in this inner regio n is also discussed.