Europa's Optical Aurora

HST/STIS image of Europa's oxygen aurora, averaged across all five visits and both 6300 and 6364 ̊A and smoothed to bring out coherent structures. Europa's north pole is up, and more emission is seen over the trailing/dusk side. The images were not stretched to match Europa's changing radius before being combined, and the dashed circle represents the average size of Europa over the five visit dates.

Auroral emissions provide opportunities to study the tenuous atmospheres of Solar System satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability.

Far-UV aurorae have been used for decades to study the atmospheres of the galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 ̊A arising from the metastable O(1D) state, observed with the Keck I and Hubble Space Telescopes while Europa was in eclipse by Jupiter on six occasions in February-April 2018.

The disk-integrated O(1D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the OI 1356 ̊A brightness on average. The ratio of emission at 6300/5577 ̊A is diagnostic of parent molecule; the 5577 ̊A emission was not detected in our dataset, which favors O2 as the dominant atmospheric constituent and rules out an O/O2 mixing ratio above 0.35. For an O2 atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of 1-9×1014 cm−2.

Katherine de Kleer, Michael E Brown
(Submitted on 12 Sep 2018)

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1809.04617 [astro-ph.EP] (or arXiv:1809.04617v1 [astro-ph.EP] for this version)
Submission history
From: Katherine de Kleer
[v1] Wed, 12 Sep 2018 18:04:55 GMT (1274kb,D)

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