Observations of solar wind ion charge exchange in the comet Halley coma

Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) observations of solar wind ions show charge exchange effects and solar wind compositional changes in the coma of comet Halley. As the comet was approached, the He²⁺ to proton density ratio increased from 2.5% in the solar wind to ∼4% about 1 hr before closest approach after which time it decreased to ∼1%. Abrupt increases in this ratio from 2.5% to 4.5% were also observed in the beginning and near the end of the so-called Mystery Region (8.6-5.5 × 105 km from the comet along the spacecraft trajectory). These abrupt increases in the density ratio were well correlated with enhanced fluxes of keV electrons as measured by the Giotto plasma electron spectrometer. The general increase and then decrease of the He²⁺ to proton density ratio is quantitatively consistent with a combination of the addition of protons of Cometary origin to the plasma and loss of plasma through charge exchange of protons and He²⁺. In agreement with the solar wind proton and He²⁺ observations, solar wind oxygen and carbon ions were observed to charge exchange from higher to lower charge states with decreasing distance to the comet. The more abrupt increases in the He²⁺ to proton and the He²⁺ to O⁶⁺ density ratios in the mystery region require a change in the solar wind ion composition in this region, while the correlation with energetic electrons and properties of the exit from this region indicate processes associated with the comet.