Short-timescale Spatial Variability of Ganymede’s Optical Aurora

Ganymede’s auroras are the product of complex interactions between its intrinsic magnetosphere and the surrounding Jovian plasma environment and can be used to derive both atmospheric composition and density. In this study, we analyzed a time series of Ganymede’s optical auroras taken with Keck I/HIRES during eclipse by Jupiter on 2021 June 8 UTC, one day after the Juno flyby of Ganymede. The data had sufficient signal-to-noise in individual 5 minute observations to allow for the first high-cadence analysis of the spatial distribution of the optical aurora brightness and the ratio between the [O i] 630.0 and 557.7 nm disk-integrated auroral brightnesses—a quantity diagnostic of the relative abundances of O, O₂, and H₂O in Ganymede’s atmosphere. We found that the hemisphere closer to the centrifugal equator of Jupiter’s magnetosphere (where electron number density is highest) was up to twice as bright as the opposing hemisphere. The dusk (trailing) hemisphere, subjected to the highest flux of charged particles from Jupiter’s magnetosphere, was also consistently almost twice as bright as the dawn (leading) hemisphere. We modeled emission from simulated O₂ and H₂O atmospheres during eclipse and found that if Ganymede hosts an H₂O sublimation atmosphere in sunlight, it must collapse on a faster timescale than expected to explain its absence in our data given our current understanding of Ganymede’s surface properties.