Ion composition and dynamics at comet Halley
Schwenn, R.; Altwegg, K.; Balsiger, H.; Geiss, J.; Young, D. T.; Ip, W. -H.; Goldstein, B. E.; Goldstein, R.; Buhler, F.; Huntress, W. T.; Lazarus, A. J.; Meier, A.; Neugebauer, M.; Rettenmund, U.; Rosenbauer, H.; Ungstrup, E.; Ghielmetti, A. G.; Sharp, R. D.; Shelly, E. G.
Switzerland, United States
Abstract
The ion mass spectrometer aboard the Giotto spacecraft measured the composition and velocity distributions of cometary ions at distances of ~7.5×l06 to ~1,300 km from the nucleus of comet Halley. Well outside the bow shock, pick-up cometary H+ ions were found in a diffuse shell-like distribution. Heavier ions (C+, H2O+-group, CO+ and S+) with similar distributions have been identified at <=3×l05 km. Solar-wind He2+ was found throughout the coma to as close as ~5,000 km; He+ produced by charge exchange was seen inside ~2×105 km. Deeper within the coma the main cometary hot-ion species identified were H+, H2+, C+, O+, OH+, H2O+, H3O+, CO+ and S+. A pile-up of heavy cometary ions was found at ~104 km from the nucleus. Giotto crossed the contact surface at ~4,600 km, based on changes in ion flow velocity and temperature. Inside, ion temperatures as low as ~340 K and outflow velocities of ~1 km s-1 were found. Outside the contact surface ion densities vary as r-2, with a transition to an r-l dependence approximately at the contact surface. A large C+ abundance throughout the coma indicates an unexpected direct source of atomic carbon. The nitrogen abundance, on the other hand, is relatively low.