Physics and chemistry of ions in the pile-up region of comet P/Halley.

Abstract

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 8000km from the nucleus (Balsiger et al. 1986a; Krankowsky et al. 1986). The maximum ion density was observed at a distance of 12000km from the nucleus rather than at closest approach (~1000km). This unexpected phenomenon, called the ion pile-up, could not be explained quantitatively so far. A new physicochemical model was developed with the aim to understand the processes which lead to the formation of this pile-up. The semi-empirical model is also used for interpreting the ion density data between the contact surface and a cometocentric distance of 50000km. A quantitative interpretation of the measured ion densities was so far possible only inside the contact surface as the physical and chemical processes are less complex there than on the outside. The model presented here has been applied to the water group ions (mass/charge 17, 18, and 19amu/e) and shows good agreement with the measurements if a neutral ammonia abundance of 1 to 1.5% relative to water is taken into account. The maximum in the H_3_O^+^-density at a distance of 12000km is the result of an increase in the electron temperature with increasing cometocentric distance, which reduces the ion recombination by electrons. As H_3_O^+^ is the most abundant ion inside 25000km this is also the reason for the enhancement of the total ion density. Although ammonia is destroyed with a scale-length of 4300km, there is a significant contribution of NH_3_^+^ to the ions with mass/charge 17amu/e in the pile-up region. At these cometocentric distances, NH_3_^+^ results from protonation of NH_2_ which is produced from ammonia by photodissociation and is relatively long-lived.