ELECTRON-PLASMA ENVIRONMENT AT COMET GRIGG-SKJELLERUP - GENERAL OBSERVATIONS AND COMPARISON WITH THE ENVIRONMENT AT COMET HALLEY

The three-dimensional electron spectrometer of the Reme plasma analyze r-complete positive ion, electron and ram negative ion measurements ne ar comet Halley (RPA-COPERNIC) experiment aboard the Giotto spacecraft , although damaged during the comet Halley encounter in March 1986, ha s provided very new results during the encounter on July 10, 1992, wit h the weakly active comet Grigg-Skjellerup (G-S). The main characteris tic features of the highly structured interaction region extending fro m approximately 26,500 km inbound to approximately 37,200 km outbound are: (1) a broad inbound ''bow wave transition'', (2) a cometosheath c haracterized by strong oscillations of the electron fluxes with peak-t o-valley ratios up to 20:1; the levels of oscillations are much larger and generally more coherent than at comet Halley or at comet Giacobin i-Zinner, despite the much lower neutral ps production rate, (3) a ''m ystery'' region, identified by significant fluxes of hundreds of eV el ectrons, and a ''mysterious' transition, about halfway between the bow wave transition/bow shock regions; they are observed as for comet Hal ley, confirming their large importance in the solar wind-comet interac tion at 1 AU, (4) an inner region which coincides with the magnetic pi leup region; at closest approach, the cold cometary electron fluxes di splay a prominent maximum, and (5) a sharp outbound bow shock and its possibily associated electron foreshock. These results are compared to the results obtained by the same instrument during the Giotto comet H alley fly-by. Despite the large difference in the size of the interact ion regions (approximately 60,000 km for G-S, approximately 2000,000 k m for Halley) due to 2 orders of magnitude difference in cometary neut ral ps production rate, there are striking similarities in the solar w ind interactions with the two comets.