The Ionosphere of Uranus as Revealed by JWST

Abstract The first spectroscopic observations of Uranus by the James Webb Space Telescope were obtained in January 2023. Using these observations, we explored the physical properties of the planet's ionosphere through the analysis of near‐infrared spectra. We found that both the northern and southern aurora were present; confirmed by localized column density enhancements. The median ionospheric temperature across the disk was 415 13 K, which is at least 300 K lower than measured by Voyager 2 in 1986, making these the lowest temperatures ever recorded. Surprisingly, auroral temperatures are not elevated in the south, and are only enhanced by tens of Kelvin in the north, indicative of limited heating or very efficient transport away from the auroral regions. Significant non‐auroral ionospheric structure is observed, including a dark band aligned with the magnetic dip equator, revealing the first imprints of Uranus's magnetic field on its ionosphere.