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.

As the largest moon in the solar system, Ganymede not only resides inside Jupiter's huge magnetosphere but also possesses an intrinsic dynamo magnetic field (Kivelson et al., 1996). The co-rotating Jovian plasma overtakes Ganymede in its orbit with sub-alfvénic velocity and drives an interaction that is unique in the solar system. The internal field acts as an obstacle for the incoming plasma flow, generating plasma waves, Alfvén wings and electric currents along the magnetopause (Frank et al., 1997;Gurnett et al., 1996;Williams et al., 1997). The incoming Jovian magnetic field reconnects at the boundary of a donut-shaped equatorial volume of closed field lines that are defined by both ends connecting to Ganymede's surface (Kivelson et al., 1997). The open field lines in the polar regions connect to Jupiter at the other end and define the extent of Ganymede's magnetosphere. Near the open-closed-field line-boundary (OCFB) observations by Hubble Space Telescope (HST) revealed the presence of two auroral ovals within Ganymede's atmosphere (

Ganymede-Jupiter's largest moon-is the only known moon in the Solar System to generate its own internal magnetic field (Gurnett et al., 1996;Kivelson et al., 1996) and therefore its space environment is of high scientific interest. In part, what makes Ganymede so interesting is that its magnetic field forms a mini-magnetosphere, with field lines connected to both hemispheres, that is, "closed," embedded deep within (semi-major axis = 14.97 Jovian radii or R J ) Jupiter's magnetosphere where the Alfvénic Mach number (M A ) is < 1. Classified as a sub-Alfvénic (and thus also sub-fast-magnetosonic by definition) interaction (e.g., Saur, 2021), no bow shock develops around the moon. Additionally, another aspect that makes Ganymede particularly interesting is the phenomenon of magnetic reconnection. Unlike Earth, and other planetary magnetospheric environments, which are embedded in dynamic solar wind conditions, the upstream conditions near Ganymede are relatively steady compared to plasma convection through Ganymede's magnetospheric system (Kivelson et al., 1998), therefore, making it possible to probe the nature of magnetic reconnection under relatively steady driving conditions (Ebert

Just prior to the end of its prime mission, Juno flew by Ganymede (Hansen et al., 2022). The flyby takes advantage of Juno's advanced instrument complement to study details of the plasma, energetic particles and fields involved in the interaction between Ganymede's and Jupiter's magnetospheres. This paper focuses on plasma waves in Ganymede's magnetosphere.Galileo plasma wave and magnetic field measurements revealed the existence of Ganymede's magnetosphere during its first flyby of the moon (Gurnett et al., 1996;Kivelson et al., 1996). Additional Galileo studies included six close flybys (Shprits et al., 2018). The plasma wave observations showed a variety of emissions commonly associated with planetary magnetospheres, including whistler-mode emissions, electron cyclotron harmonics, a band at the upper hybrid frequency, broadband noise bursts at the magnetopause, and even radio emissions emanating from the moon's magnetosphere (Gurnett et al., 1996;Kurth et al., 1997).The Juno spacecraft executed a close flyby of Ganymede at 16:56 on 7 June, day 158, 2021 with a closest approach altitude of 1046 km. The trajectory approached Ganymede over its leading hemisphere or downstream from the moon relative to the co-rotational flow of Jupiter's magnetospheric plasma. The trajectory projected into the z-x and y-x planes is shown in Figure 1 using Ganymede-centered co-rotational coordinates (sometimes referred to as G PhiO ). The +z axis is parallel to Jupiter's rotational axis and the +x axis is parallel to the nominal co-rotational plasma flow. The +y axis is in the direction of Jupiter. The radius of Ganymede (R G ) is 2,631.2 km. The blue, green, and red bars denoted with the numbers 1, 2, and 3 identify regions observed in Ganymede's magnetosphere and will be used to organize the discussion of the Waves observations.