Comments on Vasyliunas' and Pontius' studies of the effects of the planetary ionosphere and of the Coriolis force on the interchange instability

Recent Galileo and Cassini plasma and magnetic field observations indicate that the centrifugal/interchange instability plays a critical role in radial plasma transport in the rapidly rotating magnetospheres of giant planets. These observations have stimulated considerable interest in understanding the development and the properties of the instability. A complete description of the interchange instability involves an electric coupling between the magnetosphere and the planetary ionosphere. Vasyliunas (1994) pointed out that when the ionosphere is ineffective at constraining interchange motions, local magnetospheric conditions govern the plasma dynamics. In that case the Coriolis force leads to a dramatic decrease in the instability growth rate according to Pontius (1997), whereas it has a neutral influence according to Ferriere et al. (2001). The purpose of the present paper is to critically reexamine Vasyliunas' and Pontius' results and investigate the origin of their discrepancies with Ferriere et al.'s results. To that end, we consider the case of a thin plasma disk in an axisymmetric, rapidly rotating magnetosphere with no field-aligned electric currents at equilibrium.