Giotto magnetic field observations at the outbound quasi-parallel bow shock of comet Halley.

Abstract

The investigation of the outbound bow shock of comet Halley using Giotto magnetometer data leads to the following results: the shock is characterized by strong magnetic turbulence associated with an increasing background magnetic field and a change in direction by 60° as one goes inward. The thickness of the quasi-parallel shock was 120000 km. The shock is shown to be a new type of shock transition called a "draping shock". In a draping shock with high β in the transsonic transition region the transsonic region is characterised by strong directional variations of the magnetic field. The magnetic turbulence ahead of the shock is characterized by k-vectors parallel or antiparallel to the average field and almost isotropic magnetic turbulence in the shock transition region. A model of the draping shock is proposed which also includes a hypothetical subshock in which the supersonic-subsonic transition is accomplished.