Context. Electromagnetic coupling of planetary moons with their host planets is well observed in our solar system. Similar couplings of extrasolar planets with their central stars have been studied observationally on an individual as well as on a statistical basis. Aims. We aim to model and to better understand the energetics of planet star and moon planet interactions on an individual and as well as on a statistical basis. Methods. We derived analytic expressions for the Poynting flux communicating magnetic field energy from the planetary obstacle to the central body for sub-Alfvénic interaction. We additionally present simplified, readily useable approximations for the total Poynting flux for small Alfvén Mach numbers. These energy fluxes were calculated near the obstacles and thus likely present upper limits for the fluxes arriving at the central body. We applied these expressions to satellites of our solar system and to HD 179949 b. We also performed a statistical analysis for 850 extrasolar planets. Results. Our derived Poynting fluxes compare well with the energetics and luminosities of the satellites' footprints observed at Jupiter and Saturn. We find that 295 of 850 extrasolar planets are possibly subject to sub-Alfvénic plasma interactions with their stellar winds, but only 258 can magnetically connect to their central stars due to the orientations of the associated Alfvén wings. The total energy fluxes in the magnetic coupling of extrasolar planets vary by many orders of magnitude and can reach values larger than 10 19 W. Our calculated energy fluxes generated at HD 179949 b can only explain the observed energy fluxes for exotic planetary and stellar magnetic field properties. In this case, additional energy sources triggered by the Alfvén wave energy launched at the extrasolar planet might be necessary. We provide a list of extrasolar planets where we expect planet star coupling to exhibit the largest energy fluxes. As supplementary information we also attach a table of the modeled stellar wind plasma properties and possible Poynting fluxes near all 850 extrasolar planets included in our study. Conclusions. The orders of magnitude variations in the values for the total Poynting fluxes even for close-in extrasolar planets provide a natural explanation why planet star coupling might have been only observable on an individual basis but not on a statistical basis.

We analyze the magnetic field perturbations observed near Jupiter's icy moon Europa by the Galileo spacecraft during the E26 flyby on 3 January 2000. In addition to the expected large‐scale signatures of magnetic fieldline draping and induction, the E26 data set contains various prominent structures on length scales much smaller than the moon's radius. By applying a hybrid (kinetic ions and fluid electrons) model of Europa's interaction with the impinging magnetospheric plasma, we demonstrate that these fine structures in the magnetic field are consistent with Galileo's passage through a water vapor plume whose source was located in Europa's orbital trailing, southern hemisphere. Considering the large‐scale asymmetries of Europa's global atmosphere alone is not sufficient to explain the observed magnetic signatures. Combined with the recent identification of a plume during the earlier E12 flyby of Galileo, our results provide strong evidence that plume activity at Europa was a persistent phenomenon during the Galileo era.

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[1] We present an analytical model of the Alfvén wing system that is generated by the interaction between the plume of Enceladus and the corotating plasma in Saturn's inner magnetosphere. Our primary purpose is to explain the orientation of the magnetic field perturbations detected in Enceladus' Alfvén wings by the Cassini magnetometer (MAG) instrument. Observational data from numerous close Enceladus flybys show both the B x and B y components (in Enceladus interaction coordinates: B x , along corotation direction; B y , toward or away from Saturn) in the center of the northern wing tube to possess a negative sign, whereas the opposite case of B x and B y being positive was observed within the southern wing. So far, none of the available models of Enceladus' magnetospheric interaction is able to reproduce this correlation between the directions of B x and B y . On the basis of the analytical calculations of Neubauer (1980Neubauer ( , 1998 and Saur et al. (1999Saur et al. ( , 2007, we demonstrate that the observed orientation of the magnetic field may arise from the presence of negatively charged dust grains in the plume of Enceladus, serving as a sink for "free" magnetospheric electrons. Although the current carried by these particles does not make a noteworthy contribution to the magnetic field distortions in the interaction region, the negative charge accumulated by them needs to be accounted for in the quasi-neutrality condition of the plasma. The depletion of magnetospheric electrons within the plume is therefore far from causing only some localized perturbations of the magnetic field, but it drastically alters the nature of the interaction: we show that this process yields a reversal in the sign of the Hall conductivity, thereby giving rise to the observed field signatures. By applying a modified version of the Alfvén wing model developed by Saur et al. (2007), we demonstrate that the magnetic field observations from Cassini's targeted Enceladus flybys can be understood by taking into account the influence of electron-absorbing dust grains. In contrast to what is claimed in recent literature, we therefore propose that magnetic field observations near Enceladus can be completely understood in terms of a local interaction model, i.e., that it is not necessary to consider the large-scale dynamics of the flux tubes in Saturn's magnetosphere. In addition, we provide first in situ evidence that the hemisphere coupling current system predicted by Saur et al. (2007) and the associated magnetic field discontinuities are indeed present at Enceladus. The field perturbations caused by these hemisphere coupling currents arise from the partial blockage of the Alfvén wing at the nonconducting icy crust of Enceladus. This effect needs to be taken into account when interpreting Cassini MAG data from flybys that intersected the Enceladus flux tube and can only be reproduced by models that apply adequate boundary conditions to the surface of the icy moon.Citation: Simon, S., J. Saur, H. Kriegel, F. M. Neubauer, U. Motschma...

The magnetic field of Jupiter (radius R J = 71,492 km) generates a magnetosphere that extends ∼50 R J at the sun-facing side and hundreds of R J at the sun-averted side (Joy et al., 2002). The magnetic dipole axis of Jupiter is inclined by 9.6° against its rotational axis, causing the magnetic field to "wobble" when viewed from a fixed point at the rotational equator. This wobble creates periodic variations in the magnetospheric field near Jupiter's Galilean satellites. Europa, the smallest of the Galilean satellites (radius R E = 1,560.8 km), orbits its parent planet at a distance of 9.38 R J , well within the magnetosphere. Europa possesses a dilute exosphere (e.g.,

We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiter's time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0• ± 1.3 • . Our model calculations predict a significantly stronger oscillation by 5.8Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymede's dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.

The interaction between Callisto's atmosphere and ionosphere with the surrounding magnetospheric environment is analyzed by applying a hybrid simulation code, in which the ions are treated as particles and the electrons are treated as a fluid. Callisto is unique among the Galilean satellites in its interaction with the ambient magnetospheric plasma as the gyroradii of the impinging plasma and pickup ions are large compared to the size of the moon. A kinetic representation of the ions is therefore mandatory to adequately describe the resulting asymmetries in the electromagnetic fields and the deflection of the plasma flow near Callisto. Multiple model runs are performed at various distances of the moon to the center of Jupiter's magnetospheric current sheet, with differing angles between the corotational plasma flow and the ionizing solar radiation. When Callisto is embedded in the Jovian current sheet, magnetic perturbations due to the plasma interaction are more than twice the strength of the background field and may therefore obscure any magnetic signal generated via induction in a subsurface ocean. The magnetic field perturbations generated by Callisto's ionospheric interaction are very similar at different orbital positions of the moon, demonstrating that local time is only of minor importance when disentangling magnetic signals generated by the magnetosphere‐ionosphere interaction from those driven by induction. Our simulations also suggest that deflection of the magnetospheric plasma around the moon cannot alone explain the density enhancement of 2 orders of magnitude measured in Callisto's wake during Galileo flybys. However, through inclusion of an ionosphere surrounding Callisto, modeled densities in the wake are consistent with in situ measurements.

We apply a combination of data analysis and hybrid modeling to study Callisto's interaction with Jupiter's magnetosphere during the Galileo C10 flyby on 17 September 1997. This encounter took place while Callisto was located near the center of Jupiter's current sheet. Therefore, induction in Callisto's subsurface ocean and magnetospheric field line draping around the moon's ionosphere both made nonnegligible contributions to the observed magnetic perturbations. The induction signal during C10 was obscured by plasma currents to a significant degree, in contrast to previously studied Callisto flybys. Our analysis reveals that at large distances to Callisto, its magnetic environment was dominated by field line draping, leading to the formation of Alfvén wings. Closer to the surface and in Callisto's wake, Galileo encountered a quasi‐dipolar “core region” that was partially shielded from the plasma interaction and was dominated by the induced field. When exiting this core region, the spacecraft crossed a rotational discontinuity where the magnetic field vector rotated by approximately 50°. The hybrid model is able to quantitatively explain numerous key features of the observed magnetic signatures, especially the transitions between draping‐ and dipole‐dominated regimes along the C10 trajectory. The model also reproduces the electron number density enhancement by 3–4 orders of magnitude detected in Callisto's wake, requiring a substantial ionosphere to surround the moon during C10. For flybys with nonnegligible plasma currents, comprehensive knowledge of the incident flow conditions and properties of Callisto's atmosphere is required to refine existing constraints on the subsurface ocean (conductivity, thickness, and depth) based on magnetic field data. These findings are highly relevant for the upcoming JUpiter ICy moon Explorer (JUICE) mission, which will include multiple Callisto flybys.