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Jupiter's moon Io spills about a ton of material into the Jupiter magnetosphere every second. The result is a torus of plasma surrounding the planet, making an unexpected connection between emission line nebulae and planetary geophysics.
Io's amazing volcanic activity is the root of this unusual plasma. Its elliptical orbit leads to continuous tidal flexing, melting the interior and making Io the most volcanically active body in the solar system. The prodigious volcanic activity replenishes Io's rapidly escaping atmosphere. The escaping atoms and molecules orbit Jupiter and form huge neutral clouds at Io's orbit. The clouds are eventually ionized and swept into a plasma torus encircling the planet. The plasma then overtakes Io, {\nobreak smashing} into the atmosphere at $\sim$60 km/sec and causing the very large atmospheric escape flux. The volcanos therefore fuel a positive feedback loop between ions and neutrals.
Both the escaping atmosphere clouds and the plasma torus have visible-wavelength emissions lines (e.g. [SII] 6731\AA, Na D 5890\AA). They span several arcminutes, showing considerable structure, and can be readily imaged by moderate-aperture groundbased telescopes. Thus we have the opportunity to study the inner workings of a nebula: plasma sources and sinks, and mass and energy transport processes. Furthermore, several spacecraft have flown through this nebula and obtained fields and particle measurements. Despite intense this intense study (or more probably because of it), fundamental questions have arisen about the stucture and variability of the plasma torus and neutral clouds.
We have obtained a large set of narrow-band CCD images (with John Trauger, JPL), in order to understand the nature of this complex system. A video of our observations will be shown to demonstrate the magnetosphere's very dynamic behavior. Our first major goal is to use morphological information to understand what processes shape the structure of neutral clouds and plasma torus: Why is the plasma distributed so non-uniformly around Jupiter? What escape processes create the unusual appearances of the neutral clouds? Our second goal is to monitor the temporal variability of these features to understand `global change' in the Jovian system: What are the amplitudes and timescales for variations? Can the underlying relationships with Io's volcanic activity be uncovered?
There are several important areas for future work, both in observations and theory. Improved EUV observations from space are needed, as most of the energy flow lies in this range. Increased groundbased observations of Io's atmosphere will provide a measure of a critical link between Io's volcanos and Jupiter's magnetosphere. Further theoretical work will also be required to understand the connections between the many components of the Jovian system.
