ICE observations of Comet Giacobini-Zinner

Abstract

The first spacecraft encounter with a comet took place on 11 September 1985 when the International Cometary Explorer spacecraft passed through the tail of Comet Giacobini-Zinner at a distance of 7800 km from the nucleus. It provided the first definitivein-situinformation concerning the interaction of a cometary atmosphere with the flowing solar-wind plasma, and the results of initial analyses are reviewed in this paper. Large-scale mhd aspects of the interaction largely conform to prior expectation. The flow surrounding the comet is mass-loaded and slowed by situ ionization and pick-up of heavy cometary neutrals, and the solar-wind magnetic field consequently becomes draped around the obstacle, and forms an induced magnetotail. Substantial evidence exists for the permanent presence of a weak shock lying in the subsolar mass-loaded region upstream from the comet, through whether the spacecraft itself passed through shocks on the cometary flanks remains controversial. There is no doubt, however, that a sharp boundary was observed both inbound and outbound (centred onca.09h29 and 12h20 U.T.) whose width is an energetic heavyion Larmor radius (ca.104km), where the flow is deflected away from the comet and slowed, and where the magnetic field and plasma become compressed and very turbulent. The location of this boundary is also consistent with that expected for a weak shock based upon the known Giacobini-Zinner water-molecule production rate. An unexpected feature of the interaction was the extreme levels of field and plasma turbulence, and broadband wave activity observed in the region of massloaded flow.