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We develop an analytical model of the Alfvén wings generated by the interaction between a moon's ionosphere and its sub‐Alfvénic magnetospheric environment. Our approach takes into account a realistic representation of the ionospheric Pedersen conductance profile that typically reaches a local minimum above the moon's poles and maximizes along the bundle of magnetospheric field lines tangential to the surface. By solving the equation for the electrostatic potential, we obtain expressions for various quantities characterizing the interaction, such as the number flux and energy deposition of magnetospheric plasma onto the surface, the spatial distribution of currents within the Alfvén wings and associated magnetic field perturbations, as well as the Poynting flux transmitted along the wings. Our major findings are: (a) Deflection of the magnetospheric plasma around the Alfvén wings can reduce the number flux onto the surface by several orders of magnitude. However, the Alfvénic interaction alone does not alter the qualitative shape of the "bullseye"‐like precipitation pattern formed without the plasma interaction. (b) Due to the deflection of the upstream plasma, the energy deposition onto the moon's exosphere achieves its minimum near the ramside apex and maximizes along the flanks of the interaction region. (c) Even when the ionospheric conductance profile is continuous, the currents along the Alfvén wings exhibit several sharp jumps. These discontinuities generate spikes in the magnetic field that are still observable at large distances to the moon. (d) The magnitude and direction of the wing‐aligned currents are determined by the slope of the ionospheric conductance profile. Key Points: We calculate the flow deflection, electric currents, and plasma and energy flux patterns for sub‐Alfvénic moon‐magnetosphere interactionsThe number flux of the magnetospheric flow peaks above the obstacle's ramside apex, but the energy flux achieves its minimum in that regionThe currents along the Alfven wing characteristics exhibit several sharp jumps, generating observable spikes in the magnetic field [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]