Uranus’s Hydrogen Upper Atmosphere: Insights from First HST Lyman-α Images

The upper atmospheres of giant planets play a key role in various processes such as photochemistry, plasma environment and/or solar wind interaction, magnetosphere-ionosphere coupling, atmospheric escape, interaction with ring particles, etc. Uranus is among the least explored planets in the solar system, and several aspects of its upper atmosphere are not well understood. Since, H and H₂, the primary components of its neutral upper atmosphere efficiently scatter Lyman-α (Lyα, 121.567 nm) photons, remote sensing observations at this wavelength provide an excellent tool to study its upper atmosphere. In this talk, we present the first spatially resolved images of Lyα emissions from Uranus by the Hubble Space Telescope (HST). The observations were taken within two far-UV observing campaigns in 1998 and 2011, before and after Uranus's equinox in 2007. The average intensities on Uranus's disk were 860 ± 12 R and 730 ± 12 R, respectively. The images reveal widely extended emissions, detectable up to ~ 4 Uranus Radii. We performed simulations of Lyα radiative transfer in the atmosphere and analyzed the effects of the interplanetary hydrogen and H geocorona on Uranus’s Lyα signal. We find good agreement between on-disk brightnesses from simulations and the 2011 HST observations assuming the atmosphere profile derived from Voyager 2 measurements. For 1998, we find enhanced atmospheric densities of either ambient H or H₂ by factors of 1.7 and 2.1, respectively. To match the off-disk HST brightnesses in both years, a dense exosphere of hot H is required. We find that, compared to 1998, the hot H abundance required for 2011 is lower and the H profile is more extended. This hot H population is likely to be a persistent part of Uranus’s upper atmosphere. We discuss the possible production mechanisms and sensitivity of the modeled brightnesses to the parameters of the hot H profile.