Multi-fluid MHD Modeling of Europa's Plasma Interaction: Effects of Asymmetric Density in the Neutral Atmosphere

Europa orbits Jupiter within the Jovian magnetosphere, that region of space dominated by the planetary magnetic field and by plasma originating from Jupiter's moon Io. Europa's subsurface ocean and weak atmosphere interact with Jupiter's magnetic field and magnetospheric plasma. Here we investigate the role of Europa's neutral O₂ atmosphere in the generation of Europa's ionosphere and its interaction with Jupiter's magnetosphere. We have developed a 3D multi-fluid magnetohydrodynamic (MHD) model for the plasma interaction that solves for the bulk properties of 3 ion fluids (magnetospheric O⁺, ionospheric O⁺ and O₂⁺), an electron fluid, and the electromagnetic fields near the moon. We include a static distribution of neutral O₂ that represents Europa's atmosphere and provides the neutral source for ionization and charge exchange source terms that populate the ionosphere in our simulation. We use our MHD model to demonstrate the effects of variation in the neutral atmosphere on the plasma interaction during two different flybys conducted by the Galileo mission. During the E4 flyby Europa was located outside the plasma sheet and the trailing hemisphere was sunlit, while during the E15 flyby Europa was embedded within the plasma sheet and the trailing hemisphere was in shadow. By comparing our simulation results to the in situ magnetometer and plasma data collected by the Galileo spacecraft, we show that the E4 flyby is readily modeled with a neutral atmosphere with enhanced O₂ density on the trailing hemisphere; conversely, the E15 flyby is better modeled by a neutral atmosphere that is enhanced on the leading hemisphere. This finding is consistent with results from recent modeling of Europa's atmosphere that incorporates the effects of solar illumination on Europa's spatially non-uniform atmosphere as the moon rotates (Oza et al., 2019). Our work demonstrates that the state of Europa's neutral atmosphere is a crucial factor in the generation of Europa's ionosphere and, ultimately, the magnetic field perturbations associated with the plasma interaction.