Plasma injection near Io

Plasma and field observations obtained by the Galileo spacecraft during its first close encounter with Io indicate a much stronger electrodynamic interaction than anticipated. The strength of this interaction can be explained if one assumes that a significant fraction of the Io torus plasma is ionized in the immediate vicinity of the satellite. We adopt a simple model to calculate the strength and pattern of the flow perturbation resulting from a localized equatorial ionization source with dimensions much less than the radial thickness of the torus itself. Comparison with Galileo observations suggests that, when due allowance is made for the oversimplifications in the model, the observations are consistent with the hypothesis that the torus mass loading rate is highly localized near Io, and that Galileo encountered this region of concentrated mass loading at its closest approach altitude of about 0.5 Io radius. This hypothesis may also help explain two otherwise puzzling properties of the torus that were known before the Galileo encounter, the low ion temperature (relative to corotation energy) and the existence of a bright, radially confined "ribbon" feature.