Revised Upper Atmospheric Temperatures and the Need to Understand Magnetosphere-Ionosphere-Thermosphere Interactions at Uranus 

Abstract

Background: UV occultations measured by Voyager 2 during its flyby of Uranus in 1986 detected a warm stratosphere and extremely hot thermosphere [1, 2], far in excess of solar irradiance [3, 4] or internal heating [5]. These measurements imply that Uranus has the coldest lower stratosphere and yet the hottest thermosphere of any Solar System planet [6] (see Figure 1, dashed line). Uranus has the weakest internal heat of any of the giant planets [5]. The fundamental lack of understanding about the energy balance of Uranus is an example of the “giant planet energy crisis” [7].Furthermore, the Voyager 2 UV occultation measurements and models were in stark tension with Earth-based stellar occultations observed between 1977 and 1996. In [8] and [9], we present the results of reprocessing 26 archival occultations using modern techniques. While stratospheric tensions have decreased, thermospheric tensions remain, calling into question the Voyager 2 findings. In [9], we present a new 1-D atmospheric model that successfully reproduces these results. Aims: We present a new representative temperature-pressure profile for the atmosphere of Uranus from reprocessed stellar occultation observations and 1-D atmospheric modeling [8, 9]. We discuss the energy inputs to the thermosphere, circulation necessary to explain the new profiles, and the significant, potential magnetosphere-ionosphere-thermosphere interactions at Uranus. We describe critical measurements from a likely Uranus Orbiter and Probe (UOP) mission as well as from Earth beforehand that would enable the study of the energy inputs to Uranus. Prior Results & Conclusions: We reprocessed 26 archival Uranus stellar occultations, which produced temperature-pressure profiles inconsistent with Voyager 2 UVS profiles [8, 9]. A 1-D atmospheric model comprised of radiative-convective and conduction models was developed for Uranus based on these reprocessed profiles. The model reproduces the profiles and suggests a heat sink exists in the lower thermosphere. Figure 1 shows the new reference temperature-pressure profile for Uranus in comparison to the Voyager 2 profile and profiles for other planets.We conclude that the mesopause of Uranus is likely significantly higher in altitude (~10-4 mbar) than suggested from Voyager 2 profiles (~1 mbar), consistent with the mesopause levels of the other giant planets [9, 10]. We find that the stratosphere of Uranus likely contains a nearly isothermal region, again, consistent with those of the other giant planets [9, 10]. Last, we find that the required thermospheric energy flux is tens of times the solar EUV flux, underscoring the energy crisis and motivating detailed study of energy sources in the Uranian system. Ongoing Work: Measurements of Jupiter and Saturn have identified an energy crisis similar to that of Uranus. The emerging solution involves gravity waves activity that facilitates heating of the thermospheres by inducing Rayleigh drag in the polar regions. This breaks down the Coriolis force and enables meridional transport of strong auroral [11, 12] heating. While it is likely that Uranus has significant gravity wave activity [13, 14], it is possible that the auroral inputs to Uranus occur around the entire planet and are not limited to the poles [15, 16]. Much work is underway to better understand the shape and behavior of the magnetosphere of Uranus. Gravity waves may also provide the dynamical transport needed to create the heat sink observed in our new 1-D model.Therefore, additional observations of the prevalence and properties of gravity waves are critical to extending recent modeling work from Jupiter and Saturn to Uranus. While UOP may make many such observations in the 2050s, we outline Earth-based observations that can be made prior. Upcoming Stellar Occultations: In [17], we predicted Uranus and Neptune occultations 2025 – 2035 that could be observed from the ground and/or from low-Earth orbit. Additional predictions can be found in [18]. The most promising Uranus events occur in 2025, 2031, and 2032. We will describe how we intend to observe the 2025 occultation from ground-based and airborne assets, as well as other upcoming observing campaigns. We will present simulated results for temperature profiles of Uranus as well as gravity wave detections. The Shadow Chaser: In [17], we outlined the case for a low-Earth orbit small satellite to observe stellar occultations to better constrain the temperature and density profiles of the upper atmosphere of Uranus and to detect gravity waves. This mission concept, called the Shadow Chaser, is being explored at NASA Langley Research Center. By observing above the atmosphere, the Shadow Chaser could observe during the day and would not be impacted by scintillation, greatly increasing signal-to-noise. We will present simulations showing the Uranus profiles that would result from the Shadow Chaser observing the 2031 Uranus event, during which Uranus will occult a 4th magnitude K-band star.