Uranus’ hydrogen upper atmosphere: Insights from pre- and post-equinox HST Lyman-α images

¹ Royal Institute of Technology (KTH), Stockholm, Sweden
² Southwest Research Institute, San Antonio, TX, USA
³ Central Arizona College, Coolidge, AZ, USA
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France
⁵ Aix Marseille Université, CNRS, CNES, LAM, Marseille, France

^★ Corresponding author; sushenj@kth.se

Received: 14 May 2024
Accepted: 12 November 2024

Abstract

We present the first spatially resolved images of Lyman-α (Lyα) emissions from Uranus taken by the Hubble Space Telescope (HST). The observations were carried out using HST’s Space Telescope Imaging Spectrograph instrument as part of two far-ultraviolet (FUV) observing campaigns in 1998 and 2011, before and after Uranus’ equinox in 2007. The average intensities (± uncertainties) on Uranus’ disk were 860 ± 6 and 725 ± 9 R, respectively. The images reveal widely extended emissions, detectable up to ~4 Uranus radii (R_U). We performed simulations of the Lyα radiative transfer in the atmosphere, considering resonant scattering by H, Rayleigh scattering by H₂, and absorption by CH₄. We considered only solar Lyα fluxes at Uranus as the Lyα source for simulations. The effects of hydrogen in the interplanetary medium and Earth’s exosphere on Uranus’ Lyα emissions were taken into account. We find a good agreement between on-disk brightnesses from simulations and the HST observations assuming the (H, H₂, and CH₄) atmosphere profile derived from Voyager 2 measurements. Only slight adjustments of the H or H₂ densities were required in some of the simulation cases, in particular, for the 1998 observations. To match the off-disk HST brightnesses in both years, a substantial exosphere of gravitationally bound hot H is required, which we modelled assuming the hot H number density has a Chapman profile. We find that compared to 1998, the hot H abundance required for 2011 is lower and the inferred hot H profiles seem to be more extended. This bound hot H is likely to be a persistent part of Uranus’ upper atmosphere and is distinct from the escaping hot H population derived from Voyager 2 observations. We discuss the possible production mechanisms involving solar EUV radiation and study the sensitivity of the modelled brightness to the parameters of the hot H profile. We find that solar EUV radiation is not a sufficient source to explain the hot H in the exosphere of Uranus.