Hybrid Simulations of the Plasma Interaction with Europa's Atmosphere

Europa's atmosphere is about 100 times more tenuous than Io. The weaker ambient magnetic field and lower density of incident plasma means that the electrodynamic interaction is also weaker. Consequently, substantial fluxes of torus ions might reach the icy surface and produce radiolytic reactions. Molecular O2 is the dominant atmospheric product of this surface sputtering. Observations of oxygen UV emissions (specifically the ratio of OI 1356A / 1304A emissions) are consistent with an atmosphere that is composed predominately of O2 with a small amount (2%) of atomic O. Galileo observations along flybys close to Europa have revealed the existence of induced currents in a conducting ocean under the icy crust. They also showed that, from flyby to flyby, the plasma interaction is very variable. Asymmetries of the plasma density and temperature in the wake of Europa were also observed and still elude a clear explanation. Galileo mag data also detected ion cyclotron waves, which is an indication of heavy ion pickup close to the moon. We model the interaction between the plasma torus and Europa's atmosphere with a hybrid code, where ions are treated as kinetic particles moving under the Lorentz force and electrons as a fluid leading to a generalized formulation of Ohm's law. We prescribe an O2 atmosphere with a vertical density column consistent with UV observations and model the plasma properties along several Galileo flybys of the moon. We compare our results with the magnetometer observations, PLS electron density observations and a new re-analysis of the PLS plasma measurements (ion density, temperature and bulk flow velocity).