A magnetohydrodynamical model of the inner coma of comet Halley

Abstract

The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of comet Halley. The location of the boundary of this diamagnetic cavity is determined by a balance between an inward magnetic pressure gradient force and an outward ion‐neutral drag force, associated with collisions between the outwardly flowing neutrals and the stagnated ions. A one‐dimensional, time‐dependent, magnetohydrodynamical model has been developed for the inner coma of comet Halley, and includes ion‐neutral collisions, photochemical production and loss of plasma, and finite conductivity. This model is used to investigate the plasma dynamics in the vicinity of the diamagnetic cavity boundary surface. A narrow transition layer with enhanced plasma density is shown to exist just inside the boundary, although a full understanding of this layer will require a two‐ or three‐dimensional MHD model. The flux of cometary ions into this shocklike layer is removed by electron‐ion recombination. The thickness of this layer is determined by the Mach number of the incident flow.