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 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 of planetary bodies with their surrounding magnetized plasma can often be described with the magnetohydrodynamic (MHD) equations, which are commonly solved by numerical models. For these models it is necessary to define physically correct boundary conditions for the plasma mass and energy density, the plasma velocity, and the magnetic field. Many planetary bodies have surfaces whose electrical conductivity is negligibly small and thus no electric current penetrates their surfaces. Magnetic boundary conditions, which consider that the associated radial electric current at the planetary surface is zero, are difficult to implement because they include the curl of the magnetic field. Here we derive new boundary conditions by a decomposition of the magnetic field in poloidal and toroidal parts. We find that the toroidal part of the magnetic field needs to vanish at the surface of the insulator. For the spherical harmonics coefficients of the poloidal part, we derive a Cauchy boundary condition, which also matches a possible intrinsic field by including its Gauss coefficients. Thus, we can additionally include planetary dynamo fields as well as time-variable induction fields within electrically conductive subsurface layers. We implement the nonconducting boundary condition in the MHD simulation code ZEUS-MP using spherical geometry and provide a numerical implementation in Fortran 90 as supporting information on the JGR website. We apply it to a model for Ganymede's plasma environment. Our model also includes a consistent set of boundary conditions for the other MHD variables density, velocity, and energy. With this model we can describe Galileo spacecraft observations in and around Ganymede's minimagnetosphere very well.

Most of what is known from the interaction of Ganymede with Jupiter's magnetosphere originates from six Galileo close flybys (ranging from 264.4 to 3,104.9 km in altitude) (e.g., Kivelson et al., 2022).Approaching its 34th perijove, Juno came as close as 1,046 km from Ganymede's surface (sub-spacecraft latitude of 23.6°N) on 7 June 2021 (Hansen et al., 2022). Juno approached Ganymede from southern latitudes, passed through a part of the wake region that was unexplored by previous missions, through its magnetosphere to closest approach on the night side, then to the dayside, and went back into the plasma disk toward Jupiter. Highlights of the flyby from Juno's suite of instruments are reported in this issue.In this paper, we summarize plasma observations made by the Jovian Auroral Distributions Experiment (McComas et al., 2017). JADE consists of two electron (JADE-E) and one ion (JADE-I) sensors. JADE-E are top-hat analyzers measuring 0.032-32 keV electron distributions at 1 s time resolution. JADE-I has a top-hat analyzer and a time-of-flight (TOF) section to determine ion energy-per-charge (E/q) and mass-per-charge (m/q)

On 7 June 2021, Juno‐UVS mapped Ganymede's auroral emissions near a closest approach altitude of 1,046 km. The high spatial resolution map exhibits bright, 200–1,000 R, oxygen emissions organized into northern and southern auroral ovals. Though the map has incomplete global coverage, UVS observed longitudinal structure similar to that described by McGrath et al. (2013), https://doi.org/10.1002/jgra.50122 and latitudinal and vertical structure never before resolved. The mapped auroral emissions (a) display an intense narrow auroral curtain with a sharp poleward boundary, (b) have a more slowly decreasing equatorial edge on the leading hemisphere, (c) appear to originate near the surface with a vertical extent of 25–50 km, and (d) are slightly brighter in the north than the south. Additionally, we present UVS observations from the more distant Juno Ganymede flyby on 20 July 2021. We describe the observations, compare them to previous Hubble Space Telescope observations and current model predictions of the open‐closed‐field line‐boundary.