The Io mass-loading disk: Model calculations

The observations of ion cyclotron waves up to 0.5 R-J beyond the orbit of Io are best explained by the presence of a thin disk of fast neutrals whose ionization and pickup provide the free energy for the waves. We extend the model of Wilson and Schneider [1999] in order to explain the observed properties of this mass-loading region, especially the most recent Galileo observations near Io. In the extended model, some of the molecules of sulfur compounds in lo's exobase are first ionized by photoionization, impact ionization, and charge exchange. These charged particles are accelerated in the corotation electric field associated with the motion of the magnetized Io torus plasma that is moving through its exosphere. After a period of acceleration the heavy ions are neutralized by charge exchange with other exospheric neutral particles or combined with local electrons. These newly neutralized particles continue with high velocities similar to those of their former charged state, moving only under the influence of the gravity fields of Jupiter and Io, not affected by the electric and magnetic field. If they do not impact lo or its atmosphere, these neutral particles can propagate large distances across the magnetic field before they are reionized. Eventually, the reionized particles are lost by dissociation. Characteristic Io mass-loading particle distributions, such as high torus plasma density outside Io's orbit and the lower density inside, and the directional feature of the mass-loading neutral cloud, are qualitatively reproduced in the model. Meanwhile, the configuration of the mass-loading region, in which the ion cyclotron waves are observed, is obtained, and the results are consistent with Galileo wave observations. Three parameters are found to control the structure of the neutral and ion loading disks: the characteristic lifetimes of the initially created ions, of the neutral molecules, and of the ions generated by neutral particle reionization. In addition, particle source geometry, gravity, and the near Io field configurations play important roles in the Io mass-loading process.