Sulfur-bearing Ions in the Ionosphere of Comet Halley

Abstract

This paper presents the radial density profiles of the ions corresponding to 35, 45, 47, and 47 amu, inside the ionopause of Comet Halley, obtained by the heavy ion analyzer PICCA on board the Giotto spacecraft. A photochemical model of the outflowing atmosphere/ionosphere, which emphasizes the sulfur chemistry in an H2O-dominated atmosphere, is used to support the identification of the four ions as H3S(+), HCS(+), H3CS(+), and SO(+), respectively. Both the observed values and the radial profiles of the ion densities are in fair agreement with the observations in the inner portion of the ionosphere. It is suggested that the increased ion densities just inside the ionopause is assocated with the existence of a thin 'inner'shock layer also just inside the ionopause, where both lateral flow and dissociative recombination play important roles in determining the ion densities.