Europa's Plasma Interaction with Jupiter's Magnetosphere: Characterizing Variability of the Plasma Interaction with Multi-Fluid MHD Simulations

Abstract

Europa, one of Jupiter’s Galilean moons, is embedded in the region of space dominated by Jupiter’s magnetic field known as Jupiter’s magnetosphere. The interaction of Jupiter’s magnetospheric plasma and magnetic field with Europa’s atmosphere, ionosphere, surface, and subsurface ocean is affected by a variety of external and internal factors. This dissertation investigates the variability of Europa’s magnetic and plasma environment through the development and application of a multi-fluid magnetohydrodynamics (MHD) model. Our multi-fluid MHD model simulates the major plasma populations at Europa, including Jupiter’s magnetospheric plasma and the major plasma species from the moon’s ionosphere, and self-consistently solves for perturbations to the local electromagnetic fields while accounting for key mass-loading and momentum-loading processes at Europa. The model has been used to simulate various Galileo mission flybys of Europa, and was validated through comparisons of the magnetic field and plasma data, indicating that the model is suitable to apply to more general investigations of Europa’s plasma interaction and its variability. To characterize the variability of Europa’s plasma interaction caused by changes in the conditions of Jupiter’s magnetosphere, we have conducted a series of simulations using different upstream parameters that span the known range of external conditions at Europa. By separately tracking multiple ion fluids, we quantified the access of the Jovian magnetospheric plasma to Europa's surface and determined how that access is affected by changing magnetospheric conditions. We found that changes in the external conditions resulting from Jupiter’s tilted plasma sheet relative to Europa’s orbit lead to significant variations in the amount and spatial distribution of Jupiter’s magnetospheric plasma precipitating onto Europa’s surface. The total precipitation rate of the thermal magnetospheric ions increases with the density of the ambient plasma ranging between (1.8 – 26) × 1024 ions/s. Because sputtering of Europa’s icy surface by the thermal plasma is an important contributor to the generation of its atmosphere, the variations in the plasma precipitation as revealed by our modeling results provide quantitative constraints for future models for Europa’s atmosphere. We have also investigated the effects of Europa’s atmosphere on its plasma interaction by conducting a parametric study in which the atmosphere model was systematically varied to quantitatively assess the role of atmosphere density and scale height in controlling Europa’s plasma interaction. We found that variations of the atmosphere within reasonable constraints can result in increases of the density of Europa’s ionosphere by several orders of magnitude. The atmosphere also has a strong influence on the precipitation rate of Jupiter’s magnetospheric plasma, which decreases with increasing column density of the atmosphere in relatively weak atmosphere cases and then levels off for strong atmospheres at a rate governed by the density of the upstream plasma. The studies undertaken for this dissertation have provided quantitative characterization of variability of Europa’s plasma interaction in response to the external conditions of Jupiter’s magnetosphere as well as the internal influences from Europa’s atmosphere. Looking into the future, the development of our multi-fluid MHD model and its continued application to Europa provide a critical method to study this fascinating Ocean World in preparation of NASA’s Europa Clipper mission, which launches in 2024 and will travel to the Jupiter system over the next decade.