Diffusion in the Io plasma torus and its relation to the torus spatial structure.

Abstract

This is a study of the plasma diffusion processes relevant to the physical nature of the Io plasma torus at Jupiter. A knowledge of the diffusion processes involved in the Io plasma torus is essential to an understanding of the spatial structure and energetics of the torus. The only published theory of Io torus plasma diffusion, centrifugally driven flux tube interchange instability, is based on turbulent plasma interchange instability. We have examined physical properties that lead us to conclude that flux tube interchange diffusion is not a valid mechanism in the plasma torus. The collisional nature of the hot torus plasma is seen through its observed EUV emissions which dominate the energy loss from the system. Further, the torus plasma parameters fall in the range of values satisfying the criteria for the use of collisional transport theory to derive a collisional diffusion coefficient. The collisional nature of the torus plasma is characterized in the long mean free path regime where classical transport theory breaks down. We study the Chapman-Enskog method of calculating the plasma diffusion coefficient from a solution of the Boltzmann equation. Simplifying approximations of a fully ionized plasma dominated by Coulomb elastic charged particle collisions are made to derive an ad hoc non-classical diffusion coefficient which results in slow differential diffusion rates for the various sulfur and oxygen ions in the plasma torus. The radial spatial structure and energetics of the plasma torus is modeled by employing the collisional diffusion coefficient in a computer model calculation of collisional ionization-diffusive equilibrium and energy branching. The computer model employs the known significant plasma reactions involving the torus sulfur and oxygen species, utilizing the most recently available atomic parameters. In view of the failure of Neutral Cloud Theory to adequately power the copious amounts of UV radiation emitted by the Io plasma torus, we employed the radial plasma model to investigate this "energy crisis." Toward this end, we investigate the application to our plasma model of a proposed heterogeneous source of energetic electrons and a proposal of inward diffusing energetic outer-magnetospheric OII and SII ions as ad hoc heat inputs to the plasma torus electrons, in order to maintain a steady state energy balance.