Comet Halley | Published:

Ion composition and dynamics at comet Halley

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.

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