Search

Abstract We examine the dynamics of energetic magnetospheric ions in the highly perturbed and asymmetric electromagnetic environment of the Jovian moon Callisto. The Alfvénic interaction of the (nearly) corotating magnetospheric plasma with Callisto's ionosphere and induced dipole generates electromagnetic field perturbations near the moon, the structure of which vary as a function of Callisto's orbital position. For this study, these perturbations are obtained from the AIKEF hybrid model (kinetic ions, fluid electrons) which has already been successfully applied to Callisto's local plasma environment (Liuzzo et al., 2015, 2016, 2017). To isolate the influence of Callisto's ionosphere and induced dipole field on energetic ion dynamics, we analyze the trajectories of energetic hydrogen, oxygen, and sulfur ions exposed to various configurations of the locally perturbed electromagnetic fields. We present spatially resolved surface maps that display accessibility of Callisto to these ion populations for select energies from 1 keV to 5000 keV. The Alfvénic plasma interaction with (i) Callisto's induced magnetic field, (ii) Callisto's ionosphere, and (iii) the combination of both, all leave distinct imprints in these accessibility patterns. Draping of the magnetospheric field around Callisto's ionosphere partially shields the moon's trailing (ramside) hemisphere from energetic ion impacts, and the induced field tends to focus energetic ion impacts near Callisto's Jupiter-facing and Jupiter-averted apices. Depending on the nature of Callisto's Alfvénic plasma interaction, the accessibility of its surface to energetic protons may evolve non-monotonically with increasing energy. We also present maps of energetic ion accessibility and the resulting energy deposition onto Callisto at the time of the Galileo C3, C9, and C10 flybys. Our findings show that the shielding of Callisto's surface from energetic ion impacts is most effective during flybys that took place while the moon was located at an intermediate distance to the center of Jupiter's magnetospheric current sheet. In this case, the ionosphere and induced dipole field both make substantial contributions to the electromagnetic field perturbations near Callisto. [ABSTRACT FROM AUTHOR]

Using hybrid simulations and analytical calculations, we investigate the observable magnetic perturbations during the 12 planned Callisto flybys of the JUpiter ICy moons Explorer mission. During four of these encounters, Callisto will be embedded within Jupiter's magnetospheric current sheet. In these cases, Callisto's Alfvén wings and ramside magnetic field pileup will partially obscure any magnetic signatures associated with induction in a conducting layer at the moon, thereby severely complicating attempts to further constrain properties of a possible subsurface ocean. During one of these flybys, the plasma interaction will even generate magnetic signatures that are qualitatively similar to an induced field from the moon's interior. In this case, highly accurate measurements of the upstream flow parameters and Callisto's ionosphere are required to disentangle the induction signal from plasma effects. For the remaining eight encounters, Callisto's plasma interaction is expected to be sufficiently weak for an unobstructed observation of the moon's inductive signature. Plain Language Summary: Jupiter's moon Callisto may possess a salty water ocean beneath its icy surface. Due to time variations of Jupiter's magnetic field near Callisto's orbit, electric currents would be induced in such an ocean, generating magnetic field perturbations detectable outside of the moon. Therefore, magnetometer observations near Callisto can be used to prove the existence and constrain the properties of such a subsurface ocean. However, Callisto is also embedded within Jupiter's magnetosphere and is continuously exposed to a flow of plasma particles that rotate synchronously with the planet. The deflection of this plasma around Callisto generates additional electric currents and associated magnetic perturbations that may obscure the induced field from Callisto's interior. Based on modeling of these plasma currents, we demonstrate that during several flybys of the upcoming JUpiter ICy moons Explorer mission, Callisto's induction signal will be buried by plasma effects beyond recognition. These plasma signatures may even look similar to induced fields and may therefore lead to a false positive identification of the ocean. Our work constrains flyby geometries that are suitable to detect water reservoirs beneath the surfaces of Jupiter's icy moons and is highly relevant for the successful planning of synergistic measurements during the JUpiter ICy moons Explorer mission. Key Points: We present a model of Callisto's magnetic environment during the 12 upcoming flybys of the JUICE spacecraftInduction signatures from Callisto's interior will likely be obscured by Alfven wings during several flybysPlasma interaction will generate ambiguities in interpretation of JUICE magnetometer observations [ABSTRACT FROM AUTHOR]

We apply a combination of analytical modeling, hybrid simulations, and data analysis techniques to provide a comprehensive study of magnetometer data from four Galileo flybys of Callisto (C21, C22, C23, and C30) that have never been discussed in the literature before. Callisto's distance to the center of Jupiter's magnetospheric current sheet varied considerably from flyby to flyby. Therefore, the relative strength of the magnetic field perturbations due to Callisto's plasma interaction with Jupiter's magnetosphere and induction within Callisto's subsurface ocean drastically changed as well. During C21, a strong magnetic field perturbation along the corotation direction was detected in Callisto's geometric plasma shadow. This enhancement can be explained with Callisto's steady state plasma interaction only, if the upstream flow possessed a nonnegligible component away from Jupiter. During C22, Galileo only grazed Callisto's Alfvén wings which were elevated out of the flyby plane due to the ambient magnetospheric field orientation. During C23, the combination of an inclined flyby trajectory and finite gyroradius effects caused Callisto's observed Alfvén wings to be slightly asymmetric between both hemispheres. During C30, a discontinuity with a surface normal pointed toward Jupiter was detected within Callisto's geometric plasma shadow, similar to the earlier C10 flyby. Due to strong plasma interaction and an unfavorable flyby geometry (C21), a large closest approach altitude (C22), or weak inducing field (C23 and C30), no discernible induction signatures were observed during these four flybys. Based on data from all available Galileo flybys, we determine requirements on future flyby geometries that must be satisfied for an identification of Callisto's subsurface ocean in magnetometer data. [ABSTRACT FROM AUTHOR]

We examine the dynamics of energetic magnetospheric electrons exposed to the highly perturbed and asymmetric plasma environment of Jupiter's moon Callisto. The interaction of the (nearly) corotating magnetospheric plasma with Callisto's ionosphere and induced dipole locally generates intense electromagnetic pileup and draping signatures which vary as a function of the moon's distance to the center of Jupiter's magnetospheric current sheet. In our study, these field perturbations are represented using output from the AIKEF hybrid (kinetic ions, fluid electrons) model of Callisto's interaction with the corotating plasma. In order to constrain the influence of Callisto's variable electromagnetic environment on the dynamics of energetic electrons, we trace the trajectories of more than 6.7 million test particles as they travel through a distinct configuration of the locally perturbed fields. We present spatially resolved maps that display the accessibility of Callisto to electrons at energies E between 10 1 keV ≤ E ≤ 10 5 keV for multiple sets of these perturbed fields, corresponding to select distances of the moon to the center of the Jovian current sheet. The electromagnetic field perturbations near Callisto play a crucial role in generating inhomogeneous precipitation of energetic electrons onto the top of the moon's atmosphere. In particular, Callisto's Jupiter-facing and Jupiter-averted hemispheres are partially protected from energetic electron precipitation: when located far above or below the center of Jupiter's current sheet, Callisto's induced dipole shields the apices of these hemispheres, whereas near the center of the sheet, strong field line draping protects these regions. In contrast to this, the apex of Callisto's trailing hemisphere is exposed to intense energetic electron precipitation at any distance to the center of the Jovian current sheet. We also present maps of energetic electron accessibility during the Galileo C3, C9, and C10 flybys, and calculate the intensity of energetic electron flux onto multiple locations at the top of Callisto's atmosphere. These non-uniform electron fluxes likely cause inhomogeneous ionization of Callisto's atmosphere and may even contribute to irregular erosion of the surface. This paper is a companion to Liuzzo et al. (2019), who studied the dynamics of energetic ions near Callisto. • Callisto's perturbed electromagnetic environment strongly affects energetic electron dynamics. • Induced fields and Alfvén wings partially shield Callisto from impinging magnetospheric electrons. • Energetic electron precipitation onto Callisto's atmosphere is highly inhomogeneous. • Electrons may leave Callisto's local environment, bounce, and then return, causing asymmetric precipitation patterns. [ABSTRACT FROM AUTHOR]