Energetic Magnetospheric Particle Fluxes Onto Callisto's Atmosphere

Abstract

This study investigates how Callisto's perturbed electromagnetic environment—generated by the moon's interaction with the low‐energy Jovian magnetospheric plasma—affects the dynamics of high‐energy ions and electrons. We constrain how these perturbed fields influence the energetic particle fluxes deposited onto the top of Callisto's atmosphere between energies of 4.5 keV ≤ E ≤ 11.8 MeV. We use a hybrid simulation to model the variability in Callisto's perturbed electromagnetic environment over a synodic period by considering three representative scenarios of the moon's plasma interaction, corresponding to various distances of the moon to the Jovian magnetospheric current sheet. The local field perturbations are maximized near the center of the sheet (forming, e.g., signatures of field‐line pileup, draping, and Alfvén wings) whereas far from the sheet, a mere superposition of the moon's induced dipole with the background field largely explains the perturbations. We then apply a test‐particle approach to investigate the dynamics of energetic electrons and ions (protons, oxygen, and sulfur) while exposed to these fields. Since electron gyroradii are smaller than Callisto, the field perturbations generate small‐scale non‐uniformities in their flux patterns onto the moon, while the ion flux patterns are more homogeneous. Energetic electrons dominate the number flux onto the atmosphere, whereas ions dominate the energy flux. Over a synodic period, the flux patterns onto Callisto's exobase closely resemble those when the moon is near the current sheet center, since the differential energetic particle fluxes in the ambient plasma decrease by an order of magnitude when the moon travels far outside of the sheet.

Surface‐Plasma Interactions at Europa in Draped Magnetospheric Fields: The Contribution of Energetic Electrons to Energy Deposition and Sputtering

“…To solve Equation 1, we apply the numerical method of Vay (2008), which is based upon the leapfrog algorithm and is of second-order accuracy. This method has been applied in previous GENTOo simulations to calculate relativistic electron dynamics at Callisto (Liuzzo et al, 2019a(Liuzzo et al, , 2022 and Ganymede (Liuzzo et al, 2020).…”

“…For such a bouncing electron, we calculate the azimuthal displacement r az , that is, the distance in the x direction by which the electron is displaced during a half-bounce period. To determine r az , we proceed analogous to Regoli et al (2016) and Liuzzo et al (2019a); Liuzzo et al (2020Liuzzo et al ( , 2022 and employ the approach of Roederer (1967). This method uses the pitch angle of a bouncing electron when it exits Europa's interaction region to compute the distance the particle travels between the moon and its mirror point.…”

“…Thus, the factor with which to weight each of the four system III longitudes studied here cannot be precisely constrained. Previous studies have found that weighting each position differently does not noticeably alter the qualitative features of the particle influx maps, as long as the weights differ by less than a factor of approximately 5 (e.g., Addison et al, 2021;Liuzzo et al, 2020Liuzzo et al, , 2022. We therefore proceed analogous to Addison et al (2021) and weight all four cases equally in our averaging procedure.…”

“…Preceding studies by Liuzzo et al. (2019a, 2022) also revealed that electron precipitation patterns onto Callisto are substantially affected by the moon's plasma interaction. Specifically, Liuzzo et al.…”

“…In a follow‐up study, Liuzzo et al. (2022) systematically examined electron influx patterns at Callisto over the course of a synodic rotation. They found that the stretching of the moon's magnetic pileup region into the anti‐Jovian hemisphere (due to large pickup ion gyroradii) partially deflects impinging electrons away from this region when Callisto is embedded in the Jovian plasma sheet.…”