Stability of a cometary ionosphere/ionopause including pressure effects

The stability analysis by Ershkovich, McKenzie, and Axford of a cometary ionosphere/ionopause determined by ion-neutral friction is extended to include the effects of finite plasma pressure. It is shown that, from a formal viewpoint, the inclusion of pressure effects leaves intact the sufficient condition for instability. Application of the results to Comet Halley demonstrates that plasma pressure reduces the instability growth rate of the cavity and therefore that the boundary should not be expected to exhibit any significant mixing arising from overturning. The generalization to include plasma pressure is applicable to a wider range of physical parameters than is available at Halley and is of interest to other comets, particularly during periods of high solar activity. 12 refs