Multi-Fluid MHD Simulations of Europa's Plasma Interaction: Effects of Variation in Europa's Atmosphere.

Europa's plasma interaction is inextricably coupled to its O 2 atmosphere by the chemical processes that generate plasma from the atmosphere and the sputtering of magnetospheric plasma against Europa's ice to generate O 2 . Observations of Europa's atmosphere admit a range of possible densities and spatial distributions (Hall et al., 1998, https://doi.org/10.1086/305604). To better understand this system, we must characterize how different possible configurations of the atmosphere affect the 3D magnetic fields and bulk plasma properties near Europa. To accomplish this, we conducted a parameter study using a multi-fluid magnetohydrodynamic model for Europa's plasma interaction (Harris et al., 2021, https://doi.org/10.1029/2020ja028888). We varied parameters of Europa's atmosphere, as well as the conditions of Jupiter's magnetosphere, over 18 simulations. As the scale height and density of Europa's atmosphere increase, the extent and density of the ionosphere increase as well, generating strong magnetic fields that shield Europa's surface from impinging plasma on the trailing hemisphere. We also calculate the precipitation rate of magnetospheric plasma onto Europa's surface. As the O 2 column density increased from (1-2.5) × 10 14  cm -2 , the precipitation rate decreased sharply then leveled off at 2 × 10 24 ions/s for simulations with low magnetospheric plasma density and 6.4 × 10 24 ions/s for simulations with high magnetospheric plasma density. These results indicate that the coupling between Europa's plasma populations and its atmosphere leads to feedback that limits increases in the ionosphere density.