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 ( T e less than about 10 4 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.