Io's neutral clouds: From the atmosphere to the plasma torus.

摘要

Since the discovery of sodium thirty years ago, observations of Io's neutral features have provided essential insight into understanding the relationship between the Io's atmosphere and the Io torus, a ring of plasma encircling Jupiter. In this thesis I use observations and models of lo's corona, extended neutral clouds, and fast sodium jet to probe the interactions between the atmosphere, torus, and neutral clouds.; A corona and neutral cloud model, based on the model of Wilson and Schneider (1999), has been developed to study neutral loss from Io. Neutrals are ejected from Io's exobase and their trajectories followed under the influence of gravity until lost into the plasma torus. I also developed description of the plasma torus based on Voyager and ground-based observations to accurately determine neutral lifetimes.; Mutual eclipsing events between Galilean satellites were used to measure the shape of lo's sodium corona, revealing a corona that is only approximately spherically symmetric around Io. I discovered a previously undetected asymmetry: the sub-Jupiter corona is denser than the anti-Jupiter corona. Modeling implies that sodium source from the sub-Jupiter hemisphere must be twice as large as from the anti-Jupiter hemisphere.; The Galileo spacecraft has imaged a remarkable atmospheric escape process occurring in Io's ionosphere. Electrodynamic consequences of Io's motion through Jupiter's magnetosphere drive mega-amp currents through lo's ionosphere; some sodium ions carrying this current are neutralized as they leave the atmosphere. The Galileo images show that the resulting fast sodium jet removes ∼5 × 1025 atoms sec−1 from Io's atmosphere. The source region of the jet is much smaller than Io itself implying that the ionosphere is densest near Io's equator.; A model-based comparison of the neutral oxygen and sodium clouds details differences in the morphologies and spatial extent of each: sodium extends only 1/4 the way around Jupiter while oxygen forms a complete torus. Sodium emission results from resonant scattering making it relatively easy to determine sodium abundance. The oxygen intensity is highly dependent on the highly variable plasma torus. This complicates the analysis of oxygen observations since the nature and magnitude of plasma variations has not been well characterized.