We analysed far-ultraviolet (FUV) spectra of Uranus obtained by the HST STIS and COS instruments in 2012 and 2014, respectively, to determine the brightness of Raman-scattered Lyman-alpha (Ly α ) emissions centred at 1280 Å (hereafter, the Raman feature). The Raman feature is unique among the Solar System’s giant planets and forms in Uranus’ atmosphere due to weak vertical mixing of hydrocarbons with H 2 , leading to efficient Rayleigh–Raman scattering. Methane is the dominant hydrocarbon species on Uranus, and since it absorbs FUV radiation, it affects the Rayleigh–Raman scattering of Ly α photons by H 2 and, eventually, the brightness of the Raman feature. We derive a brightness of 20 −6 +1 R from the STIS data, which is similar to the brightness measured by Voyager 2 UVS during the 1986 flyby of Uranus, when considering the suggested recalibration of UVS measurements by a factor of ∼0.5. Based on the observed brightness, we constrain the upper altitude (pressure) level for the abundance of methane in the upper atmosphere using radiative transfer simulations that include resonant scattering by H, Rayleigh–Raman scattering by H 2 , and absorption by CH 4 . We considered the solar Ly α flux as the source of Ly α radiation at Uranus. We find that resonant scattering by H significantly affects Rayleigh–Raman scattering by H 2 and thus the modelled brightness of the Raman feature. We derive methane profiles by obtaining the simultaneous fit to the observed Ly α , as well as the 1280 Å brightness of Uranus. Methane appears to be depleted (number density becomes less than 1 cm −3 ) above the altitude (pressure) range of ∼478–515 km (4 × 10 −3 –2.4 × 10 −3 mbar), while the Ly α absorption optical depth reaches unity for methane in the altitude (pressure) range of ∼237–257 km (2.54 × 10 −1 –1.65 × 10 −1 mbar). When neglecting resonant scattering by H, the methane depletion must be deeper in the atmosphere at an altitude (pressure) of ∼395 km (1.4 × 10 −2 mbar), similar to previous findings based on Voyager 2 observations of the feature. The analysis of the Raman feature provides independent CH 4 constraints in the upper atmosphere for detailed photochemistry modelling and highlights the importance of UV instruments for the future Uranus Orbiter and Probe (UOP) mission.

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 (RU). 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.

We use the Magnetospheric Multiscale mission (MMS) to study electron acceleration at Earth's quasi-perpendicular bow shock to address the long-standing electron injection problem. The observations are compared to the predictions of the stochastic shock drift acceleration (SSDA) theory. Recent studies based on SSDA predict electron distribution being a power law with a cutoff energy that scales with upstream parameters. This scaling law has been successfully tested for a single Earth's bow shock crossing by MMS. Here we extend this study and test the prediction of the scaling law for seven MMS Earth's bow shock crossings with different upstream parameters. A goodness-of-fit test shows good agreement between observations and SSDA theoretical predictions, thus supporting SSDA as one of the most promising candidates for solving the electron injection problem.

We use the Magnetospheric Multiscale mission (MMS) to study electron acceleration at Earth’s quasi-perpendicular bow shock to address the long-standing electron injection problem. The observations are compared to the predictions of the Stochastic Shock Drift Acceleration (SSDA) theory.   Recent studies based on SSDA predict electron distribution being a power law with a cutoff energy that scales with upstream parameters. This scaling law has been successfully tested for a single Earth's bow shock crossing by MMS. Here we extend this study and test the prediction of the scaling law for seven MMS Earth's bow shock crossings with different upstream parameters. A goodness-of-fit test shows good agreement between observations and SSDA theoretical predictions, thus supporting SSDA as one of the most promising candidates for solving the electron injection problem.