The James Webb Space Telescope built the first three-dimensional map of Uranus's ionosphere — the charged atmospheric layer where auroras form. The instrument was NIRSpec, pointed at the ice giant's upper atmosphere to measure temperature, ion density, and emission intensity as functions of altitude and latitude.
The results disagree with every existing model. Ion densities peak near one thousand kilometers above the cloud tops but are significantly weaker than predicted. Temperatures peak higher, around three to four thousand kilometers, creating a vertical separation between the density maximum and the temperature maximum that the models didn't anticipate.
Two bright auroral bands appear near the magnetic poles, which on Uranus are not near the rotational poles. Uranus's magnetic field is tilted sixty degrees from its spin axis and offset from the planet's center — one of the strangest magnetic configurations in the solar system. The auroras sweep across the surface in complex patterns dictated by this misalignment, nothing like the stable ovals of Jupiter or Earth.
Between the auroral bands sits a depleted region where both emission and particle density drop. This depletion zone appears to be a direct consequence of the magnetic field geometry — the offset dipole creates regions where charged particles are channeled away rather than toward the atmosphere.
The confirmation that matters most is temporal: Uranus's upper atmosphere has been cooling for decades. Voyager 2 measured the thermosphere temperature in 1986 and found it inexplicably hot — hundreds of degrees warmer than solar heating alone could explain. The JWST measurements show that whatever process heated it has been winding down. The energy crisis of ice giant atmospheres — where does the excess heat come from — is becoming the energy crisis of ice giant atmospheres — where did the excess heat go.
The structure of the ionosphere is the answer key to questions about energy balance in atmospheres where sunlight is too faint to explain what's observed. Every exoplanet characterization of a sub-Neptune or super-Earth ice world will eventually need this baseline.