Observability of ENA emissions at Europa and Callisto: predictions for the JUICE mission

We analyze the emission of energetic neutral atom (ENA) flux from Callisto and Europa as a tool to probe moon-plasma interactions on a global scale. In situ ENA detectors sample a two-dimensional snapshot of the entire interaction region, as opposed to observations that provide magnetic field and plasma data only along one-dimensional trajectories. Charge exchange between energetic magnetospheric ions and cold atmospheric neutrals results in ENAs that propagate along rectilinear trajectories. Since the distribution of ENA flux is resultant from the interaction between the ambient plasma, the magnetospheric field configuration and the neutral gas distribution, ENA images can contextualize and quantitatively constrain these aspects of the moon-magnetosphere interaction on a local as well as a global scale. We combine the perturbed electromagnetic fields from a hybrid plasma model with a particle tracing tool to model ENA generation for the energetic ions interacting with Europa's and Callisto's neutral envelopes. By taking into account the point-like size (on scales of the plasma interaction) and limited field of view of a spacecraft detector, we apply our model to investigate which features of the emitted ENA flux will be observable by the JUICE spacecraft during its close flybys of both moons.