OBSERVATION OF INTERPLANETARY PARTICLES IN A COROTATING INTERACTION REGION AND OF ENERGETIC WATER GROUP IONS FROM COMET GRIGG-SKJELLERUP

During the flyby of the Giotto S/C with comet Grigg-Skjellerup (G-S) ( 9-11 July 1992) energetic particles (E-H2O > 60 keV, E-p > 30 keV, E-a lpha > 3.5 MeV), magnetic fields and plasma ions were measured by thre e different detectors on board Giotto. The flyby period was characteri zed by high magnetic field magnitudes (> 12 nT) and an enhanced backgr ound flux caused most likely by the reverse shock of a corotating inte raction region (CIR). Within the Cir three different particle accelera tion events were detected: 1. Proton and alpha-particle acceleration ( E-p > 4.5 MeV, E-alpha > 3.5 MeV) caused by a sudden increase of the m agnetic field combined with a change in direction. 2. Proton and H2O-i on (E-p > 220 eV, E-H2O > 260 keV) acceleration caused by a directiona l discontinuity of the magnetic field. 3. Acceleration of water group ions in the foreshock and bow shock regions of comet G-S associated wi th a further increase of the magnetic field magnitude. Particle fluxes , magnetic field measurements, pitch angles and energy spectra are use d to discuss the particle acceleration processes such as shock drift m echanism, inductive acceleration, Fermi acceleration and adiabatic com pression. The results are compared with those obtained during the Giot to-Halley encounter. (C) 1996 Published by Elsevier Science Ltd.