ENERGETIC IONS AT COMET GRIGG-SKJELLERUP MEASURED FROM THE GIOTTO SPACECRAFT

ON 10 July 1992, the Giotto spacecraft flew within about 200 km of the nucleus of comet Grigg-Skjellerup; this is only the third comet at wh ich in situ measurements have been made, and the encounter constituted the closest approach so far of a spacecraft to a cometary nucleus. He re we report the detection by the EPONA instrument on Giotto of charge d, energetic particles deep within the inner coma of Grigg-Skjellerup. In contrast to previous spacecraft encounters with comets Giacobini-Z inner (in 1985) and Halley (in 1986), well defined, periodic intensity variations recorded in the particle fluxes suggest that the ions clos e to the nucleus were strongly coupled to the ambient magnetic field. The present data indicate that Giotto new on the nightside of the nucl eus.