On the formation of Io's inner torus

In December 1995 and November 2002, the Galileo spacecraft traversed the inner Io plasma torus on the J0 and A34 passes, respectively. Observations of electron density showed a steep drop off at the inner boundary located between 4.5RJ and 5RJ on the two passes, and a much more gradual decrease in the outer torus. The [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243-26260, 2001] model of torus formation considers multiple ionization and neutralization stages for the pickup ions, and predicts an inner torus boundary location which depends on the local plasma velocity at Io. For an inner torus boundary at ∼4.7RJ, ions are picked up into a plasma flowing at ∼37 km/s (74 km/s is the corotation velocity at 5.9RJ). The steepness of the boundary and the double peaked density structure seen on the two Galileo passes is not reproduced by the [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243-26260, 2001] model, which considers the source of mass loading at Io to be uniformly distributed around the moon and does not include outward radial convection. Adding this convection to the model, as would be required to maintain a steady state magnetodisk, steepens the inner torus boundary, but does so over very long timescales. The different inner torus density structure observed on the J0 and A34 passes is not reproduced by either the [Wang, Y.-L., Russell, C.T., Raeder, J. The Io mass loading disk: model calculations. Journal of Geophysical Research 106, 26243-26260, 2001] or outward radial convection models and indicates the possible occurrence of a large mass loading event in the 7 years between the passes or longitudinal asymmetry in the torus structure.