Effects of Europa's Atmospheric Asymmetries on its Plasma Interaction: Insights from the Galileo E15 Flyby

To understand the large-scale effects of asymmetries in Europa's neutral atmosphere on its plasma interaction, we have conducted multi-fluid magnetohydrodynamic simulations of the Galileo E15 flyby. Europa's neutral atmosphere is intrinsically coupled to the plasma interaction between Europa and Jupiter's magnetosphere through various mass-loading and momentum-loading processes. Therefore global-scale variations in the neutral atmosphere are expected to cause variability in the plasma interaction. Several sources in the literature offer evidence and explanations for time-varying asymmetries in the atmosphere, but models for the plasma interaction have yet to investigate the effect of this variation on Europa's ionosphere and the resulting perturbations to the magnetic and plasma environment. We have therefore developed a series of multi-fluid MHD simulations (Harris et al., 2021) for the E15 flyby considering different models of atmospheric asymmetries, including an atmospheric bulge placed on Europa's leading hemisphere as a result of solar illumination. Our simulations indicate that such an atmospheric asymmetry during the E15 flyby can well explain both the measured magnetic field and plasma density variations in Europa's wake. Our results are consistent with the analysis of Volwerk et al. (2001), who identified a source of mass-loading on Europa's leading hemisphere during the E15 flyby. These results are also consistent with the atmospheric models of Plainaki et al. (2013) and Oza et al. (2019), who predicted that an atmospheric bulge should form on Europa's leading hemisphere during configurations similar to that of the E15 flyby. Our results demonstrate the crucial importance of taking into account variations in Europa's neutral atmosphere in understanding Europa's plasma interaction and have important implications for developing high-fidelity plasma interaction models for Europa.