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The Long-term Cooling of Uranus’ Upper Atmosphere: Update from the 2022 Apparition

Presentation #323.01 in the session Uranus and Neptune Systems (Poster)

Published onOct 23, 2023
The Long-term Cooling of Uranus’ Upper Atmosphere: Update from the 2022 Apparition

With an orbital period of 84 years, Uranus’ season averages 21 years long. Equinox last occurred in 2007 as the IAU North pole came into view. The solar forcing passed through a seasonal minimum at equinox; and owing to Uranus’ high obliquity, this minimum was quite pronounced. By contrast, solar forcing produces a pronounced maximum at solstice, due in 2031 – slightly delayed by the proximity of aphelion. These respective epochs of net cooling and heating affect the temperature of Uranus’ upper atmosphere – the thermosphere and ionosphere – but with a delayed response owing to the radiative time constant of these upper layers, which - unlike the troposphere - is much shorter than an orbital period. This response would be sooner if there were also a dynamical process for transporting heat in these layers. A candidate for such a process is that which produces the excess heating in the upper atmospheres of all the giant planets, and which cannot be explained by radiation alone. This has been an open question since the Voyager epoch. Answering it for Uranus could explain it for all. If the seasonal response of Uranus’ upper atmosphere can be treated as an insolation-forced oscillating system, damped by radiation to space, its characteristic response time can be determined by the phase lag of the temperature minimum following equinox. We have been monitoring the temperature of Uranus’ neutral thermosphere and ionosphere through the near-IR ro-vib bands of H2 and H3+, respectively. The temperature of Uranus’ upper atmosphere has been in decline since 1992, when observations began. As of the 2022 apparition, there has been no sign of a seasonal reversal in the 15 years since equinox, despite the mounting solar forcing. The maximum phase lag for such an oscillating system is one season (infinite time constant), 9 apparitions away. We update the time series using the results of our monitoring campaign for the 2022 apparition.

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