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

Abstract

AbstractThis 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.