The foreshock region upstream from the Comet Halley bow shock

A few hours prior to the crossing of the Comet Halley bow shock, the Giotto spacecraft intermittently encountered an electron foreshock region. The electron foreshock is characterized by magnetic connection to the cometary bow shock and increased field aligned electron heat flux directed away from the bow shock. A similar region was intermittently encountered by the ICE spacecraft prior to its crossing of the Giacobini-Zinner bow wave. During periods of magnetic connection with the Halley bow shock, enhanced magnetic field fluctuations were observed. These enhancements are interpreted as indirect evidence of an ion foreshock in the electron foreshock. No clearly identifiable backstreaming protons are observed during these periods of magnetic connection, however, because it may be difficult to separate a backstreaming population from the cometary pick-up proton population already present in the upstream region.