Ion Pickup and Velocity Space Thermalization at Outer Planet Moons: Wave-Particle Interactions

Ion pickup at the active moons of the outer planets is a fundamental process in which newly created ions from moon exospheres interact with the ambient corotating plasma and are accelerated to co-move with the background flow. Recent spacecraft observations reveal strong electromagnetic wave activity likely generated by this pickup process. In this study, we investigate the ion pickup process at Europa and Io using the hybrid VPIC code, in which ions are treated kinetically while electrons are modeled as a massless fluid. In the moon’s rest frame, ambient ions initially stream at the corotating velocity perpendicular to the background magnetic field relative to stationary pickup ions, resulting in the two populations being clustered at opposite gyro-phases. This configuration simultaneously excites transverse electromagnetic ion cyclotron waves and compressional magnetic fluctuations associated with mirror and ion-Bernstein modes, with amplitudes reaching a few percent of the background field. Using field–particle correlation analysis, we demonstrate how these waves scatter ions in velocity space, enabling newly created ions to be efficiently picked up and leading to isotropization of the distribution function in both gyro-phase and pitch angle. We identify three key parameters that control the instability threshold: the Alfvén Mach number and plasma beta of the ambient corotating ions, and the ambient-to-pickup ion density ratio. A comprehensive parameter survey is performed to determine the instability threshold. This study advances a kinetic understanding of ion pickup and provides a framework for interpreting spacecraft observations at the moons of the outer planets.