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Jupiter's satellite auroral footprints are a consequence of the interaction between the Jovian magnetic field with co‐rotating iogenic plasma and the Galilean moons. The disturbances created near the moons propagate as Alfvén waves along the magnetic field lines. The position of the moons is therefore "Alfvénically" connected to their respective auroral footprint. The angular separation from the instantaneous magnetic footprint can be estimated by the so‐called lead angle. That lead angle varies periodically as a function of orbital longitude, since the time for the Alfvén waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfvén Wing auroral spots collected by Juno‐UVS during the first 43 orbits, this work provides the first empirical model of the Io, Europa, and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfvén travel times between the three innermost Galilean moons to Jupiter's northern and southern hemispheres are estimated from the lead angle measurements. We also demonstrate the accuracy of the mapping from the Juno magnetic field reference model (JRM33) at the completion of the prime mission for M‐shells extending to at least 15 RJ. Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon‐induced decametric emissions. Plain Language Summary: The interaction between the Jovian magnetospheric plasma and the Galilean moons gives rise to a complex set of phenomena, including the generation of auroral spots magnetically related to the moons and the generation of radio emissions. The magnetic perturbations local to the moons propagate at a finite speed along the magnetic field lines, and reach the northern and southern Jovian hemispheres where they produce the auroral spots. Studying the position of these auroral spots and how they vary over a complete Jovian rotation provides information about the magnetic mapping, as they map directly to the actual physical positions of the moons. The magnetic field model derived from Juno's prime mission is in good agreement with the observation of the satellite footprints. This paper provides information about how the electromagnetic perturbation resulting from the interaction propagates at a finite speed to create auroral spots, leading to an angular shift between the instantaneously magnetically‐mapped position of the moon and the auroral footprint, a quantity also known as the "equatorial lead angle". The present work provides an empirical fit of the equatorial lead angle for Io, Europa, and Ganymede derived from Juno data. Key Points: Over 1,600 ultraviolet spectral images of the Io, Europa, and Ganymede footprints from Juno are analyzedEmpirical formulae for the Io, Europa, and Ganymede equatorial lead angles derived from Juno data are providedAlfvén travel time estimates are derived, constraining the Alfvénic interaction at the three innermost Galilean moons [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Observations of Ganymede by Juno‐UVS during a close flyby (during Juno's 34th Jupiter orbit) captured unique high‐spatial‐resolution measurements of Ganymede's auroral emissions. Organized into two polar ovals, the positions and intensities of the auroral emissions are consistent with previous Hubble Space Telescope observations. The observed morphology of the auroras on the leading hemisphere of Ganymede exhibits latitudinal structure never before resolved. Previous studies suggest the poleward edge of the emissions trace the poleward most position of the magnetic field lines that have both ends rooted to Ganymede. We show that a magnetic field model of Ganymede within Jupiter's larger magnetosphere predicts last‐closed field‐lines very close to the observed auroral emissions, as expected. More distant observations taken during Juno's following orbit (orbit 35) capture auroral emissions at slightly different longitudes. They too show similar agreement with previous observations and current magnetic field models. Key Points: The high spatial resolution observations revealed auroral emissions of over 1000 Rayleighs, brighter than previously observedThe leading hemisphere aurora exhibits an intense auroral curtain with a sharp poleward boundary and more slowly tapering equatorial edgeJuno‐UVS observed Ganymede's auroral emissions extending up to a maximum of 50 km altitude [ABSTRACT FROM AUTHOR]

Ionospheric conductance contributes to regulate the characteristics of the ionospheric current and the closure of the magnetosphere-ionosphere circuit in the ionosphere. Measurements of Birkeland currents with the Juno magnetometer have indicated that they are statistically larger in the south. It has been suggested that these asymmetries may be the consequence of a higher Pedersen conductance in the southern hemisphere. We have derived the local precipitated electron energy flux and their characteristic energy from 14 multi-spectral images collected with the ultraviolet spectrograph on board Juno. This information was then used as input to an ionospheric model providing the density of H3+, H+, and hydrocarbon ions to calculate the spatial distribution of the Pedersen conductance for Juno perijoves 1-15. We show that the area-integrated conductance is closely proportional to the H3+ ion content and quasi equal in the north and the south. The mean conductance is 0.47 mho in both hemispheres. However, local variations in the Pedersen conductivity and/or hemispheric differences between the magnetospheric rotation and the rotation velocities of the neutrals can also result in asymmetry of Birkeland currents. [ABSTRACT FROM AUTHOR]

Jupiter’s polar auroral region hosts UV auroral emissions that relate to the magnetospheric dynamics from the outer magnetosphere. Juno-UVS has discovered intriguing features characterized by expanding emission circles of UV-brightness <140 kR. These events are located at the border of the previously defined swirl region, nearby the polar dark region. The features expand into a circular shape up to ∼1,000 km in radius, at expansion velocities from 3.3 ± 1.7 up to 7.7 ± 3.5 km/s, as measured over the four best observed cases. Using color ratio measurements as a proxy for the depth of the recorded features, the mean electron energy responsible for these emissions is 80–160 keV. Events occurring in the outer magnetosphere at distances >100RJ are likely causing for these features. Dayside magnetopause reconnection and Kelvin-Helmholtz instabilities resulting from the shear flows near the magnetopause are expected to generate field-aligned currents that could potentially be the cause of these features [ABSTRACT FROM AUTHOR]

Integrating simultaneous in situ measurements of magnetic field fluctuations, precipitating electrons, and ultraviolet auroral emissions, we find that Alfvénic acceleration mechanisms are responsible for Ganymede's auroral footprint tail. Magnetic field perturbations exhibit enhanced Alfvénic activity with Poynting fluxes of ~100 mW/m2. These perturbations are capable of accelerating the observed broadband electrons with precipitating fluxes of ~11 mW/m2, such that Alfvénic power is transferred to electron acceleration with ~10% efficiency. The ultraviolet emissions are consistent with in situ electron measurements, indicating 13 ± 3 mW/m2 of precipitating electron flux. Juno crosses flux tubes with both upward and downward currents connected to the auroral tail exhibiting small‐scale structure. We identify an upward electron conic in the downward current region, possibly due to acceleration by inertial Alfvén waves near the Jovian ionosphere. In concert with in situ observations at Io's footprint tail, these results suggest that Alfvénic acceleration processes are broadly applicable to magnetosphere‐satellite interactions. Plain Language Summary: Jupiter's moon Ganymede interacts with the planet's rapidly rotating magnetic field, which generates an aurora in the Jovian upper atmosphere. The Juno spacecraft crossed magnetic field lines connected to this aurora. We found that a specific type of wave, similar to a wave produced when a string is plucked, is responsible for accelerating the electrons sustaining this aurora. This type of interaction between a moon and the planet it orbits is likely a common process occurring at other exoplanetary systems. Key Points: First in situ particles and fields measurements connected to Ganymede's auroral tail are reportedAlfvén wave activity is observed with Poynting fluxes of ~100 mW/m2 capable of accelerating electrons into the atmosphereGanymede's footprint tail contains electron populations consistent with Alfvénic acceleration and precipitating energy fluxes of ~11 mW/m2 [ABSTRACT FROM AUTHOR]