Alternating north-south brightness ratio of Ganymede’s auroral ovals: Hubble Space Telescope observations during the Juno PJ34 flyby

We report results of Hubble Space Telescope (HST) observations from Ganymede’s orbitally trailing side which were taken around the flyby of the Juno spacecraft on June 7, 2021. We find that Ganymede’s northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter’s magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede’s aurora is the momentum of the Jovian plasma sheet north and south of Ganymede’s magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and associated electromagnetic energy fluxes carrying the power driving the auroral emission. No clear statistically significant time-variability of the auroral emission on short time scales of 100s could be resolved. We show that Ganymede’s auroral emission at OI 1356 Å requires electron energy fluxes of several tens of mW m−2.