Densities and abundances of hot cometary ions in the coma of P/Halley

On its flight by P/Halley, the Giotto spacecraft carried a high-energy range spectrometer (HERS) for measuring the properties of cometary ions picked up by the solar wind in the nearly collisionless regions of the coma. Preliminary estimates of the ion densities observed by HERS have now been reevaluated and extended; density profiles along the Giotto trajectory are presented for 13 values of ion mass per charge. Comparison with the physical-chemical model of the interaction of sunlight and the solar wind with the comet by Schmidt et al. reveals that, with the exception of protons and H₂⁺, all ion densities were at least an order of magnitude higher than predicted. The high ion densities cannot be explained on the basis of compression of the plasma, but require additional or stronger ionization mechanisms. Ratios of the densities of different ion species reveal an overabundance of carbonaceous material and an underabundance of H₂⁺ compared to the predictions of the Schmidt et al. model. While the densities of solar wind ions (H⁺ and He⁺⁺) changed sharply across a magnetic discontinuity located 1.35 × 10⁵ km from the comet, this feature, which has been called both the "cometopause" and the "magnetic pileup boundary," was barely distinguishable in the density profiles of hot cometary ions. This result is consistent with the interpretation that the magnetic pileup boundary detected by Giotto was caused by a discontinuity in the solar wind and is not an intrinsic feature of the interaction of the solar wind with an active comet.