UVS Observations of Ganymede's Aurora During Juno Orbits 34 and 35

Juno UVS, an ultraviolet spectrograph sensitive to wavelengths 68-210 nm, performed unique observations of Ganymede’s aurora on the approach to Juno’s 34th and 35th perijoves (PJ). The combination of Juno’s 2 rpm spin rate, UVS’ 7.2° long “dog-bone” shaped slit, and the UVS scan mirror allows for the recording of 7.2° wide scans across Ganymede’s disk every 30 s. Through the wide slits we are able to capture integration times of 17 ms per spin for each resolution element in the observed swath. For the PJ34 Ganymede encounter on June 7, 2021 at 16:56:08 UTC, Juno UVS captured data during 16:52-16:56-17:04 UTC at altitudes varying from 3262-1046-6820 km. Over this time period Ganymede’s angular diameter varied from 54°-93°-33° on the sky, while the nadir solar phase angle varied from 149°-102°-25°. Juno UVS achieves a spatial resolution of ≈0.2° giving a best-case nadir spatial resolution of 4 km (0.08° Ganymede latitude). The PJ34 UVS data provide a sparse, but high-resolution look at Ganymede’s aurora, and can be used to locate the last closed field lines to an accuracy of about one degree of latitude. Our data show not only a narrow auroral curtain of emission in both the north and south, but also fainter emission extending from the auroral curtains towards the equator on Ganymede’s leading hemisphere. Additionally, the auroral curtains appear more uniform than expected from previous HST observations that show more patchy and variable emissions from observation to observation. For the PJ35 Ganymede encounter on July 20, 2021 at 16:48:30 UTC, Juno UVS captured data during 16:33-16:48-17:27 UTC. The increased PJ35 observational period relative to the PJ34 encounter is due to the larger range of the event, 52,717-49,999-64,252 km, making the angular extent of Ganymede only 5.6°-5.8-4.6° on the sky (at a nadir solar phase angle 99°-81°-43°) and the best-case nadir spatial resolution 175 km (comparable to HST imagery). UVS not only spatially resolved Ganymede, but also spectrally separates the prominent 130.4 and 135.6 nm O auroral emissions, and their brightness ratio, diagnostic of their auroral excitation mechanisms.