Giotto measurements of cometary and solar wind plasma at the Comet Halley bow shock

The interaction of comets with the solar wind depends on the ionization of the heavy cometary neutrals (mostly H2O and its dissociation products O, OH) which flow out from the nucleus, and the coupling of these newly produced cometary ions with the solar wind through its embedded magnetic field. The 'pick-up' of these heavy ions slows the solar wind such that a shock may form. As expected, the structure of such a shock transition is highly complex because the gyroradius of a heavy cometary ion is much larger than that of a solar wind proton. Here we present a comparative study of the solar wind electrons and protons and the cometary pick-up ions measured by Giotto at the inbound crossing of the bow shock at comet Halley. We find a highly structured shock transition starting with a cometary ion 'foot' seen at a distance on the order of the cometary ion gyroradius upstream from a sharp decrease in the solar wind proton speed. The total solar wind thermal and magnetic pressure is dominated by the relatively small population of cometary ions throughout the shock region.