  We report results of Hubble Space Telescope 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 electromagnetic energy fluxes ultimately powering the
auroral acceleration process. No clear statistically significant time
variability of the auroral emission on short time scales of 100s could be
resolved. We show that electron energy fluxes of several tens of mW m$^{-2}$
are required for its OI 1356 \AA$\;$ emission making Ganymede a very poor
auroral emitter.
