THE FLOW OF THE CONTAMINATED SOLAR-WIND AT COMET-P GRIGG-SKJELLERUP

On July 10, 1992, the Giotto spacecraft encountered a second comet, na mely, P/Grigg-Skjellerup. Despite the small production rate of this co met (approximately 2 orders of magnitude smaller than that of P/Halley and 1 order of magnitude smaller than that of P/Giacobini-Zinner) the plasma flow showed several distinct flow transitions. We have applied theoretical models, previously developed to explain the existence and location of the flow boundaries at comets P/Giacobini-Zinner and P/Ha lley to this new data set for comet P/Grigg-Skjellerup. The observed b ow shock crossings are used to evaluate the production rate of the com etary neutrals. Using this production rate, the position of the collis ionopause and the ionopause are estimated. The observed location of th e magnetic pile-up boundary corresponds well to the theoretical calcul ations. We also find the reason that a magnetic field-free region was not encountered was because the ionopause, while being sharp, was insi de the spacecraft trajectory which had an estimated closest approach d istance greater than 120 km. Finally we have used the observed spatial profile of the solar wind magnetic field together with a simple one-d imensional, multispecies chemical model to calculate the radial densit y profiles of the H2O group ions outside the bow shock. Comparison of the calculated total H2O group ion density profile with observations i ndicates a good fit in the inner region where the H2O+ ion dominates. While the corresponding fit in the outer region, where the O+ ion domi nates, becomes progressively less satisfactory with increasing cometoc entric distance, it is noted that the observations themselves are beli eved to be less reliable at these large distances.