Two classes of models for temporal variability of the Io torus

Abstract

The stability and temporal variability of the Io torus are investigated using spatially averaged, uniform models. Stable equilibrium torus models are found for two classes of models, one with nonlinear radial transport loss of plasma and one with linear transport. The former can be realized by centrifugally driven interchange instability, and the latter by Jovian atmospheric wind‐driven interchanges. The torus models include photoionization and recombination in addition to torus ion sputtering and other neutral injection mechanisms. Given the possible variability of the Io atmosphere, significant temporal variations may occur in the Io torus. For stable models with linear transport, significant electron cooling is predicted for ion or neutral densities more than an order of magnitude above values observed by Voyager, unless unknown electron heating mechanisms intervene. Higher densities may not be achievable. For nonlinear transport by centrifugal interchange, the torus density need not be limited by electron cooling. For either type of transport, stable torus models exist with densities smaller than Voyager values by at least a factor of 100. Observational tests are proposed that may distinguish between the two types of transport. Stability of the present Io torus does not require either nonlinear transport or centrifugally driven interchange instability.