H. Reme et al., ELECTRON-PLASMA ENVIRONMENT AT COMET GRIGG-SKJELLERUP - GENERAL OBSERVATIONS AND COMPARISON WITH THE ENVIRONMENT AT COMET HALLEY, J GEO R-S P, 98(A12), 1993, pp. 20965-20976
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.