ELECTRON DISTRIBUTIONS UPSTREAM OF THE COMET HALLEY BOW SHOCK - EVIDENCE FOR ADIABATIC HEATING

We report on three-dimensional plasma electron (22 eV to 30 keV) observations upstream of the comet Halley bow shock, obtained by the RPA-1 COPERNIC (Reme Plasma Analyzer - Complete Positive Ion, Electron and Ram Negative Ion Measurements near Comet Halley) experiment on the Giotto spacecraft. Besides electron distributions typical of the undisturbed solar wind and backstreaming electrons observed when the magnetic field line intersects the cometary bow shock, we find a new type of distribution characterized by enhanced low energy (< 100 eV) flux which peaks at 90-degrees pitch angles. These are most prominent when the spacecraft is on field lines which pass close to but are not connected to the bow shock. The 90-degrees pitch angle electrons appear to have been adiabatically heated by the increase in the magnetic field strength resulting from the compression of the upstream solar wind plasma by the cometary mass loading. We present a model calculation of this effect which agrees qualitatively with the observed 90-degrees flux enhancements.