Effect of the acceleration current on the centrifugal interchange instability

[1] The centrifugal interchange instability is the primary driver of radial plasma transport in the magnetospheres of Jupiter and Saturn. Most previous theoretical treatments of this instability have ignored the role of the acceleration current and have assumed that the divergence of the centrifugal drift current in the magnetosphere is closed by Pedersen currents in the underlying ionosphere, through the connection provided by Birkeland (magnetic-field-aligned) currents. This is a reasonable approximation when the radial transport speed is small compared with the rotation speed. However, the exponential growth of the instability inevitably leads to the eventual violation of this condition. I analyze a simplified model of the Io plasma torus to show that when the radial transport speed becomes comparable to the rotation speed, the acceleration current becomes the primary mechanism for closure (actually local cancellation) of the centrifugal drift current, and the connection to the planetary ionosphere therefore becomes irrelevant. An immediate consequence is that the growth rate of the instability does not exceed the planetary rotation rate.