Previous treatments of rotationally driven interchange motion assume rigid corotation and incorporate centrifugal force into an effective gravity. When the ionospheric acceleration time is short, this approximation is acceptable, but in remote regions of the Jovian magnetosphere, the constraining influence of the ionosphere is increasingly ineffective. Conservation of angular momentum then governs local dynamics, and the Coriolis force is an essential part of the perturbation analysis. If the ionosphere is absent, the result agrees with earlier studies of magnetized Couette flow. If the ionosphere is merely very ineffective, the interchange criterion is not changed, but the instability growth rate is significantly reduced from earlier calculations. ¿ 1997 American Geophysical Union