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Monitoring Ganymede’s UV auroral emissions using HST/COS

Presentation #501.07 in the session Icy Galilean Satellites: Magnetosphere and Exosphere (Oral Presentation)

Published onOct 23, 2023
Monitoring Ganymede’s UV auroral emissions using HST/COS

In 2021 we performed high spectral resolution observations of Ganymede’s leading and trailing hemispheres in the far UV using the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) with the G130M grating. This instrument setting encompasses the neutral oxygen auroral emissions at 135.6 nm and 130.4 nm, as well as a fainter oxygen line at 115.2 nm, which we detected for the first time on Ganymede’s leading hemisphere only. The observed disk-averaged 135.6 nm / 130.4 nm intensity ratios are consistent with emissions dominated by electron impact dissociative excitation of O2 on the leading hemisphere, with a contribution from the dissociative excitation of H2O required to reduce the ratio on the trailing hemisphere, in agreement with previous studies. However, the 135.6 nm / 115.2 nm ratio is larger than expected based on laboratory measurements of the relevant emission cross sections. We discuss the implications of this for the composition and distribution of Ganymede’s atmosphere. We also present trends in auroral brightness as a function of Jovian magnetic field geometry for both hemispheres, with our new observations filling previous gaps in system 3 coverage.

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