Magnetohydrodynamic processes in the inner coma of comet Halley

Abstract

A two‐fluid, two‐dimensional, magnetohydrodynamical (MHD) model is used to study the dynamics of the inner coma of comet Halley. Ion production, electron‐ion recombination, charge transfer collisions, and ion‐neutral friction are included. The model adopts a cylindrical grid with nonuniform radial spacing extending from a cometocentric distance of 1000 km out to 107 km. A magnetic barrier and a diamagnetic cavity both appear in the model results, and the plasma flow is directed around the cavity. Electron temperature profiles have recently been suggested for the inner coma in which large temperature enhancements (Te ≈ 25,000 K) are present at a radial distance of about 10,000 km and have been used to explain, via reduced recombination, the ion density enhancement observed by the Giotto ion and neutral mass spectrometers at this location. Using our MHD model as well as analytical methods we find that large electron temperature enhancements in the inner coma result in large reductions in the magnetic field strength which were not observed by the Giotto magnetometer. We suggest that perhaps the electron temperature is only enhanced to a value of about 10,000 K rather than to 25,000 K at the ion pileup.