Constraining the Influence of Callisto's Perturbed Electromagnetic Environment on Energetic Particle Observations

This study focuses on constraining the role that Callisto's perturbed electromagnetic environment had on energetic charged particle signatures observed during the Galileo mission. To do so, we compare data from the Energetic Particle Detector (EPD) obtained during four close encounters of the moon with a model framework that combines hybrid simulations for low-energy plasma and test-particle tracing simulations for high-energy particles. By comparing model results for energetic particle dynamics in both uniform and perturbed electromagnetic fields, we systematically disentangle the role that geometric effects (i.e., absorption of particles by Callisto's solid surface) have on observed energetic particle signatures compared to those associated with Callisto's perturbed electromagnetic environment (generated by the moon's induced magnetic field and plasma interaction currents). We show that observed flux drop-outs in the energetic ion pitch angle distributions (PADs) are largely driven by their absorption by Callisto's surface: their large gyroradii exceed the size of the moon, facilitating their impact onto the icy surface and preventing their detection by EPD. However, features observed in the energetic electron PADs can only be explained with an accurate representation of the moon's perturbed environment, since electrons closely follow the orientation of the electromagnetic fields. Our findings therefore illustrate the key role that the moon's induced field and magnetospheric plasma interaction have on the dynamics of energetic electrons, emphasizing the importance of accurately modeling Callisto's locally perturbed electromagnetic environment when attempting to interpret data from past and future encounters, including those anticipated from the upcoming JUICE mission. Jupiter's moon Callisto is continuously exposed to a combination of low-energy plasma and high-energy charged particles. As the moon interacts with these populations, its local electromagnetic field environment becomes highly perturbed. During four Callisto encounters during the Galileo mission to Jupiter, a sensor onboard the spacecraft detected clear signatures of the high-energy ions and electrons. To understand the degree to which Callisto's low-energy plasma interaction affects these signatures, we systematically model the dynamics of energetic particles as they travel through various electromagnetic field configurations. We find that the energetic ion observations are not strongly affected by the perturbed fields, but instead can be understood in terms of the particles being absorbed by Callisto's solid surface. However, electrons are highly sensitive to the draped fields, and the Galileo observations can only be understood when considering the perturbed electromagnetic environment near the moon. Our results underscore the importance of accurately modeling Callisto's perturbed environment when hoping to understand energetic particle signatures observed during the upcoming JUICE mission. We compare energetic charged particle data with a model to explain pitch angle distributions observed during four Galileo flybys of Callisto Electron flux drop-outs are shaped by the perturbed fields associated with Callisto's induced field and magnetospheric plasma interaction Observed ion fluxes can be mainly explained through geometric considerations, with Callisto's surface shadowing particles from detection