VELOCITY SHEAR IMPOUNDMENT OF THE IO PLASMA TORUS

The Io plasma torus apparently has a much longer lifetime than anticip ated on the basis of simple theoretical stability analyses. This quand ary is particularly evident at the outer edge of the torus, the plasma ramp, where the steep radial gradient in plasma content indicates the presence of some confining influence. Previous attempts to explain th is feature have focused on the possibility that high-energy particles could impound the colder, Iogenic plasma, but these studies have prove d inconclusive. We consider an alternative mechanism whereby the devel opment of an unstable perturbation is interrupted by the observed shea r in the rotational velocity. An example in simplified geometry demons trates how the relative azimuthal displacement of radially adjacent pe rturbations might eliminate their coherency and impose a finite satura tion amplitude. Fully nonlinear numerical simulations using the Rice c onvection model at Jupiter produce analogous results, suggesting that perturbations are suppressed where the shear is strong. The crucial pa rameter is the ratio of the classic, linear growth rate to a,rate that characterizes the velocity shear. The electric fields produced by uns table perturbations farther out are effectively shielded from the shea r region. We suggest that this effect helps impound the plasma torus a nd is at least partly responsible for producing the ramp.