Ganymede

Short-Timescale Spatial Variability of Ganymede’s Optical Aurora

By Keith Cowing
Status Report
astro-ph.EP
September 11, 2024
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Short-Timescale Spatial Variability of Ganymede’s Optical Aurora
Calibrated images of Ganymede’s 630.0 nm auroral emission displayed in 20 R contours. To better reveal the spatial variability we smoothed the data using a Gaussian kernel with a FWHM of 0. ′′5 approximating typical seeing conditions for the morning of 2021-06-08 UTC. The time in the upper left of each image is the UTC time at the start of the observation, the annotation in the lower right is the distance to the plasma sheet centrifugal equator; positive when Ganymede is above the mid-plane and negative when it is below. As Ganymede moved vertically through the plasma sheet, the hemisphere closest to the mid-plane exhibited the brightest aurora. From approximately 14:00 to 14:45 UTC, Ganymede is within ±0.5 RJ and the brightness is more evenly-distributed across the disk. Ganymede passed through the highestdensity plasma mid-plane at 14:20 UTC, so the top two rows display data when Ganymede was above the mid-plane (with enhanced brightness at southern latitudes) and the bottom row displays data when Ganymede was below the mid-plane (with enhanced brightness at northern latitudes). Because of the blurring effect of atmospheric seeing, the calibrated brightness distribution of the individual pixels is lower than the disk-averaged value reported in de Kleer et al. (2023) and table 1, which assume all emission originates from a disk with Ganymede’s solid-angular size. The white grid shows the physical size and orientation of Ganymede, with north pointing upward (for more details on this observing geometry, see figure 2). — astro-ph.EP

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, 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.

Zachariah Milby, Katherine de Kleer, Carl Schmidt, François Leblanc

Comments: 20 pages, 13 figures, 7 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2409.06055 [astro-ph.EP] (or arXiv:2409.06055v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.06055
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Journal reference: The Planetary Science Journal, Volume 5, Issue 7, July 2024
Related DOI:
https://doi.org/10.3847/PSJ/ad49a2
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Submission history
From: Zachariah Milby
[v1] Mon, 9 Sep 2024 20:30:53 UTC (9,658 KB)
https://arxiv.org/abs/2409.06055
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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