Io's fast sodium clouds and the escape of Io's ionosphere.

摘要

This dissertation proposes a new description of the interaction between Jupiter's magnetosphere and the atmosphere of Jupiter's volcanically active moon, Io. Several observational data sets of Io's extended sodium clouds support the hypothesis that Io's ionosphere is constantly leaking out of Io's atmosphere, leading to a host of atmospheric escape processes. This is a change from the current prevailing view of the interaction, wherein atmospheric loss is driven entirely by bombardment of Io's atmosphere by ions and electrons in Jupiter's plasma torus. It also differs with current photochemical models of Io's ionosphere, which assume that electric fields in Io's atmosphere are insufficient to drive ion currents there. This idea suggests that sodium plays a unique role in Io's atmosphere, and argues against the idea that sodium is a representative tracer for the other species (i.e., SO;Two distinct features of Io's fast sodium clouds are seen to be governed by the motion of ions recently created near Io. Escaping atomic sodium ions undergo charge exchange with sodium in Io's atmosphere, producing the 'directional feature.' Sometimes molecular ions containing sodium escape from the ionosphere, and are later dissociated in the torus to produce a feature called the 'stream.' In both cases, the gyrospeeds indicate varying amounts of plasma slow-down near Io, consistent with plasma flow near a conducting ionosphere. Also, the velocities parallel to the B-field and the orientation of the cyclotron orbits indicate surprisingly little perturbation to the B-field in Io's vicinity.;This dissertation makes predictions which are relevant to recent observations by the Galileo spacecraft. Ionospheric escape should produce a peculiar distribution of sodium pickup ions in the region near Io which was sampled by Galileo. (The relevant Galileo data has not been published at the time of this writing.) In addition, this research sets limits on any intrinsic magnetic field on Io, and suggests that alternate explanations should be investigated for the magnetic field perturbation seen by Galileo (Kivelson et al. 1996).