The upper atmospheres of Uranus and Neptune

Abstract: We review the current understanding of the upper atmospheres of Uranus and Neptune, and explore the upcoming opportunities available to study these exciting planets. The ice giants are the least understood planets in the solar system, having been only visited by a single spacecraft, in 1986 and 1989, respectively. The upper atmosphere plays a critical role in connecting the atmosphere to the forces and processes contained within the magnetic field. For example, auroral current systems can drive charged particles into the atmosphere, heating it by way of Joule heating. Ground-based observations of H$_3^+$ provides a powerful remote diagnostic of the physical properties and processes that occur within the upper atmosphere, and a rich data set exists for Uranus. These observations span almost three decades and have revealed that the upper atmosphere has continuously cooled between 1992 and 2018 at about 8 K/year, from $\sim$750 K to $\sim$500 K. The reason for this trend remain unclear, but could be related to seasonally driven changes in the Joule heating rates due to the tilted and offset magnetic field, or could be related to changing vertical distributions of hydrocarbons. H$_3^+$ has not yet been detected at Neptune, but this discovery provides low-hanging fruit for upcoming facilities such as the James Webb Space Telescope (JWST) and the next generation of 30 metre telescopes. Detecting H$_3^+$ at Neptune would enable the characterisation of its upper atmosphere for the first time since 1989. To fully understand the ice giants we need dedicated orbital missions, in the same way the Cassini spacecraft explored Saturn. Only by combining in-situ observations of the magnetic field with in-orbit remote sensing can we get the complete picture of how energy moves between the atmosphere and the magnetic field.