MODELING THE EUROPA PLASMA TORUS

The existence of a torus of plasma generated by sputtering from Jupite r's satellite Europa has long been suspected but never yet convincingl y demonstrated. Temperature profiles from Voyager plasma observations indicate the presence of hot, possibly freshly picked-up ions in the g eneral vicinity of the orbit of Europa, which may be interpreted as ev idence for a local plasma torus. Studies of ion partitioning in the ou ter regions of the Io torus reveal that the oxygen to sulfur mixing ra tio varies with radial distance; this may indicate that oxygen-rich ma tter is injected from a non-Io source, most probably Europa. We have c onstructed a quantitative model of a plasma torus near the orbit of Eu ropa which takes into account plasma input from the Io torus, sputteri ng from the surface of Europa, a great number of ionization and charge exchange processes, and plasma loss by diffusive transport. When the transport time is chosen so that the model's total number density in c onsistent with the observed total plasma density, the contribution fro m Europa is found to be significant although not dominant. The model p redicts in detail the ion composition, charge states, and the relative fractions of hot Europa-generated and (presumed) cold Io-generated io ns. The results are generally consistent with observations from Voyage r and can in principle (subject to limitations of data coverage) be co nfirmed in more detail by Ulysses.