Distribution of plasma in the Io plasma torus from radio occultations during the Juno epoch
Phipps, Phillip Harvey
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The innermost Galilean satellite, Io, is the most volcanically active body in the solar system. The volcanic activity leads to material being released into Jupiter's magnetosphere near Io's orbit. This material becomes ionized and trapped in Jupiter's rotating magnetic field. The trapped material forms a torus of material around Jupiter that is called the Io plasma torus. It contains an inner cold torus and an outer warm torus. In this dissertation, I determine and interpret the distribution of plasma in the Io plasma torus from radio occultation observations by the Juno spacecraft. I perform a feasibility study to show that Juno radio occultation observations should be able to detect the Io plasma torus. Based on this feasibility study, I predict that key Io plasma torus parameters -- value and location of maximum total electron content, and scale height -- can be determined with 10--20 percent uncertainties. I analyze Juno radio occultation observations from Perijove 1. Perijove is the point where the spacecraft makes its closest approach. The observations are taken for a 6 hour period around perijove and are labeled as Perijove followed by a number which corresponds to the orbit number. From the observations I determine the Io plasma torus parameters and find that inferred densities are ~30% larger than models suggested. These results show that Juno radio occultation observations can detect and usefully characterize the Io plasma torus. I analyze data from Perijoves 3, 6, and 8 and determine how Io plasma torus parameters vary. In this set of observations, the warm torus maximum total electron content and scale height do not vary greatly. I test the prediction that the torus lies in the centrifugal equator by modeling the equator location. Observed and predicted locations agree reasonably if a Juno magnetic field model and a simple current sheet model are used. I find that the contribution of the current sheet is significant, which suggests that remote observations of the location of the Io plasma torus can be used to constrain Jupiter's magnetospheric current sheet.