Electron energetics in the inner coma of comet Halley

Abstract

A quasi‐two‐dimensional model of the spatial and energy distribution of electrons in the inner coma of comet Halley has been constructed from a spherically symmetric ion density profile based on Giotto measurements, using the two‐stream electron transport method and the time‐dependent electron energy equation. A variety of heating and cooling processes were included, such as cooling via rotational, vibrational, and electronic excitation of water vapor due to electron impact. A sharp jump in the electron temperature was found to be present at a cometocentric distance of about 15,000 km. This thermal boundary separates an inner region where cooling processes are dominant from an outer region where heat transport is more important. Both thermal (Te less than about 104 K) and suprathermal electron populations exist inside the thermal boundary with comparable kinetic pressures. Outside the thermal boundary, a cold electron population does not exist, and the electrons are almost isothermal along the magnetic field lines.