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

TL;DR

This study uses Hubble observations to reveal that Ganymede's auroral ovals alternate in brightness, driven by interactions with Jupiter's plasma sheet, providing insights into the moon's magnetic and auroral dynamics.

Contribution

First direct observation of alternating auroral brightness on Ganymede linked to Jovian plasma interactions, highlighting magnetic coupling effects.

Findings

Auroral ovals alternate brightness with the hemisphere facing Jupiter's plasma sheet being brighter.

Electron energy fluxes of tens of mW/m$^2$ are needed for observed emissions.

No significant short-term variability detected in auroral brightness.

Abstract

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.