COMETARY ION PRESSURE ANISOTROPIES AT COMETS HALLEY AND GRIGG-SKJELLERUP

Cometary water group ions are the dominant population controlling the interaction of comets with the solar wind. The distribution function o f this population has important effects on the overall structure and b oundaries in the comet-solar wind interaction and on the plasma wave a ctivity produced. In this paper we present new results on the behavior of the water group ions measured by Giotto at comets Halley and Grigg -Skjellerup. The pressures perpendicular and parallel to the magnetic field are presented and the implications explored. We show that during the Halley inbound pass, the firehose instability may excite the obse rved waves with low wavenumber. Also, we find that the energy density of energetic cometary ions is greater than the observed difference bet ween the energy released from the ring-like distribution and the energ y in the pickup-excited waves. This indicates that another energy sour ce must be contributing to the acceleration, probably related to the d eceleration of the solar wind in the comet frame.