Short-Timescale Spatial Variability of Ganymede's Optical Aurora

Ganymede's aurora 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 aurora taken with Keck I/HIRES during eclipse by Jupiter on 2021-06-08 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 aurora brightness and the ratio between the 630.0 and 557.7 nm disk-integrated auroral brightnesses -- a quantity diagnostic of the relative abundances of O, O$_2$ and H$_2$O 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 O$_2$ and H$_2$O atmospheres during eclipse and found that if Ganymede hosts an H$_2$O 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.