Abstract Detections of auroral emission from Callisto have been difficult due to the low density of electrons that excite the aurora at its large orbital distance from Jupiter. However, large ground-based telescopes with sensitive spectrographs can detect the faint emissions at optical wavelengths when Callisto is eclipsed by Jupiter. In this study, we analyzed two sets of optical wavelength spectra observed with Keck I/HIRES. We detected the 630.0 nm O i emission line in individual 5 minute integrations and found its disk-averaged brightness decreased with distance from the centrifugal equator of Jupiter’s plasma sheet. The average brightness matched simulations of emission from an atmosphere composed of O, O 2 , H 2 O, and CO 2 using the most up-to-date estimates or upper limits for their individual column densities, supporting the prevailing paradigm that Callisto’s atmosphere may be the densest of the icy Galilean satellites and is composed primarily of O 2 . We also modeled the emission from the same atmosphere if an ionosphere inhibited excitation by the thermal electron population but found the resulting brightness was an order of magnitude below observations.
Milby et al. (Fri,) studied this question.
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