GIOTTO ION MASS-SPECTROMETER MEASUREMENTS AT COMET-P GRIGG-SKJELLERUP

The Giotto ion mass spectrometer high-intensity spectrometer (IMS-HIS) measured fluxes of ions from about 260,000 km before (1008:37 UT spac ecraft time) to about 86,000 km after (1701:33 UT spacecraft time) clo set approach to comet P/Grigg-Skjellerup during the encounter on July 10, 1992. Although the HIS sensor was not designed to measure protons, these ions were measured far from the comet. Close in to the comet, t he ions observed were probably also protons, although heavier ions can not be completely ruled out. Considerable temporal structure appears i n the data, well-correlated with the data of other instruments onboard , especially those of the magnetometer. In particular, the ion count r ate correlates with the direction of the magnetic field. This strong m odulation at the water group ion cyclotron period (approximately 90 s) inside the inbound bow wave indicates a very narrow ion pitch angle d istribution. Hence at Grigg-Skjellerup the ions appear to experience v ery little pitch angle scattering. This may result from strong compres sion in the rapidly increasing magnetic field.