DRIFT WAVE INSTABILITY IN THE IO PLASMA TORUS

We present a linear normal-mode analysis of low-frequency electrostatic drift waves in the Io plasma torus, providing a kinetic description of the centrifugal interchange instability of the torus regulated by coupling to Jupiter's ionosphere. We assume a periodic potential perturbation with azimuthal wavenumber m >> 1 and solve a boundary value problem to obtain the wave dispersion relation and radial eigenfunction. For given m the growing mode is a standing wave in the corotating reference frame. If the outer torus boundary is taken as a discontinuity, the growth rate is proportional to m times the torus flux tube content times the ion drift frequency, divided by the Pedersen conductivity of Jupiter's ionosphere. The inner torus boundary has a modest stabilizing effect for azimuthal wavelengths greater than the radial thickness of the torus. The finite slope of the outer torus density profile has a more pronounced stabilizing effect, reducing the growth rate by the factor 2-beta/m, where beta approximately 2 is the exponent of the assumed power law decline of flux tube content with distance. Even so, the e-folding time is of the order of 1 hour, much less than the inferred residence time of torus ions. The growth rate can be further reduced dramatically by the stabilizing effect of an as-yet unobserved ring current distribution, the "impoundment" effect proposed by Siscoe et al. (1981). Various theoretical models of global radial transport in Jupiter's magnetosphere, including corotating convection, interchange diffusion, and transient flux tube convection, can be understood as plausible nonlinear evolutions of electrostatic drift waves. Further observations and/or numerical simulations are needed to ascertain the relative importance of these transport mechanisms.