PHYSICS AND CHEMISTRY OF IONS IN THE PILE-UP REGION OF COMET P HALLEY/

Measurements by the mass spectrometers onboard Giotto during the flyby at comet P/Halley showed a steep increase in the ion density outside the diamagnetic cavity at a distance of about 8000 km from the nucleus (Balsiger et al. 1986a; Krankowsky et al. 1986). The maximum ion dens ity was observed at a distance of 12 000 km from the nucleus rather th an at closest approach (similar to 1000 km). This unexpected phenomeno n, called the ion pile-up, could not be explained quantitatively so fa r. A new physicochemical model was developed with the aim to understan d the processes which lead to the formation of this pile-up. The semi- empirical model is also used for interpreting the ion density data bet ween the contact surface and a cometocentric distance of 50 000 km. A quantitative interpretation of the measured ion densities was so far p ossible only inside the contact surface as the physical and chemical p rocesses are less complex there than on the outside. The model present ed here has been applied to the water group ions (mass/charge 17, 18, and 19 amu/e) and shows good agreement with the measurements if a neut ral ammonia abundance of 1 to 1.5% relative to water is taken into acc ount. The maximum in the H3O+-density at a distance of 12 000 km is th e result of an increase in the electron temperature with increasing co metocentric distance, which reduces the ion recombination by electrons . As H3O+ is the most abundant ion inside 25 000 km this is also the r eason for the enhancement of the total ion density. Although ammonia i s destroyed with a scale-length of 4300 km, there is a significant con tribution of NH: to the ions with mass/charge 17 amu/e in the pile-up region. At these cometocentric distances, NH3+ results from protonatio n of NH2 which is produced from ammonia by photodissociation and is re latively long-lived.