Exploring the Uranian upper atmosphere with a general circulation model

   Knowledge of Uranus’ atmosphere has been limited by the fact that only one spacecraft has to-date performed a close flyby. With the Voyager 2 mission setting much of the landscape of Uranus science since 1986 [1, 2], new remote observations steadily providing new, but mostly height-integrated information [3], numerical modelling is a key tool that allows us to provide further constraints on the ice giant.  

   Solar heating alone is not enough to heat the giant planets to their observed thermospheric temperatures which defines the so-called ‘Giant Planet Energy Crisis’ [4]. What is likely to be supplying the extra energy is the coupling between the magnetosphere and upper atmospheres of these planets. For the Gas Giants, this process has been thoroughly studied, and sufficient energy can be provided from the aurora. Modelling has addressed how this energy can be transported to explain the observed hot thermospheric temperatures of these planets at all latitudes and longitudes in despite of suppression of equatorward transport from Coriolis forces [5, 6].  

   For Uranus, and likely Neptune, the problem is intrinsically different. The relative importance of the coupling terms from solar-wind or rotation-driven magnetosphere interaction, charged particle precipitation and joule heating remains under-constrained. The planet has a strongly tilted magnetic field, potentially generating stronger diurnal and seasonal variability and dynamical energy transport. At Uranus, the observed long-term cooling of the planet also represents an additional phenomenon not seen on the other Gas Giants [7].

   A physics-based, three-dimensional, general circulation model, of Uranus’ upper atmosphere (thermosphere and ionosphere) is under development, based on previously published similar models for Jupiter and Saturn, and we use this to address some of the above questions. Naturally, this development also encourages comparative aeronomy. We discuss preliminary results from UTIM, the Uranus Thermosphere-Ionosphere Model, which self-consistently calculates winds, temperatures and composition in response to external drivers. The model allows us to place local observations into a global context and understand underlying physical processes. Due to the current unconstrained nature of the planet, parameter space can be explored to guide where Uranus may lie. We will present case-studies at different conditions, outline future refinement and feature development of the model.

References

[1] Lindal, Gunnar F., et al. "The atmosphere of Uranus: Results of radio occultation measurements with Voyager 2." Journal of Geophysical Research: Space Physics 92.A13 (1987): 14987-15001.

[2] Herbert, Floyd, et al. "The upper atmosphere of Uranus: EUV occultations observed by Voyager 2." Journal of Geophysical Research: Space Physics 92.A13 (1987): 15093-15109.

[3] Melin, Henrik, et al. "The ionosphere of Uranus as revealed by JWST." Geophysical Research Letters 52.22 (2025): e2025GL118301.

[4] Melin, Henrik. "Towards a solution to the energy crisis." Nature Astronomy 4.9 (2020): 837-838.

[5] Müller‐Wodarg, I. C. F., et al. "Atmospheric waves and their possible effect on the thermal structure of Saturn's thermosphere." Geophysical Research Letters 46.5 (2019): 2372-2380.

[6] Müller-Wodarg, Ingo CF, et al. "Temperatures of Jupiter’s Upper Atmosphere: The Role of the Planetary Magnetic Field." The Astrophysical Journal Letters 990.1 (2025): L22.

[7] Melin, Henrik. "The upper atmospheres of Uranus and Neptune." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378.2187 (2020).