JWST Discovers the Vertical Structure of Uranus' Ionosphere

Abstract On 19 January 2025, the James Webb Space Telescope (JWST) program 5073 observed Uranus for almost a full rotation with NIRSpec. We present the first ever vertical ionospheric profiles of temperature, volumetric density and total emission. Temperature profiles peak between 3,000–4,000 km at all longitudes. Densities peak near 1,000 km with values up to (4.45 0.12) × , lower than predicted by 1‐D models. We identify two bright emission regions at 50–110W and 220–290W, near the auroral regions. A distinct emission and density depletion occurs between 190–240W, possibly reflecting magnetic topology influences, analogous to ionospheric dark regions at Jupiter. These results offer new insights into Uranus's upper atmosphere, confirm its long‐term cooling trend (426 2 K), and provide key constraints for ice giant ionospheric and magnetospheric modeling.