Properties of plasma electrons in the magnetic pile-up region of comet Halley

In the environment of comet P/Halley, a sharp transition, named the cometopause, was detected by the VEGA and GIOTTO spacecraft around 170000km from the nucleus. This boundary separates the outer region of the cometosheath controlled by the solar wind from the inner region of cometary plasma. In electron measurements provided by the GIOTTO-RPA experiment, a clear discontinuity was detected 20 % closer to the nucleus than the cometopause as seen in the ion measurements and coincided with an abrupt increase of the magnetic field magnitude when the field lines began to pile up. Significant changes in the electron distribution function through this discontinuity and within the following region down to 45000 km from the nucleus are presented. The main changes noted at the electron plasma discontinuity were a decrease of the density above 10 eV and a modification of the distribution function which turned from isotropic to anisotropic. Following this discontinuity structures of different plasma characteristics were found in regions of different magnetic field directions which are throught to result from convected interplanetary magnetic field features. It is noted some resemblance between the Venus mantle and the region around the comet which is comprised between the cometopause and the ionopause