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 (10:08:37 UT spacecraft time) to about 86,000 km after (17:01:33 UT spacecraft time) closest approach to comet P/Grigg- Skjellerup during the encounter on July 10, 1992. Although the HIS sensor was not designed to measure protons, the unusual flyby geometry at Grigg-Skjellerup allowed the sensor to detect these ions. Considerable temporal structure appears in the data, well- correlated with the data of other instruments onboard, especially those of the magnetometer. In particular, the proton count rate correlates with the direction of the magnetic field.