Auroral emissions from Uranus and Neptune.

Uranus and Neptune possess highly tilted/offset magnetic fields whose interaction with the solar wind shapes unique twin asymmetric, highly dynamical, magnetospheres. These radiate complex auroral emissions, both reminiscent of those observed at the other planets and unique to the ice giants, which have been detected at radio and ultraviolet (UV) wavelengths to date. Our current knowledge of these radiations, which probe fundamental planetary properties (magnetic field, rotation period, magnetospheric processes, etc.), still mostly relies on Voyager 2 radio, UV and in situ measurements, when the spacecraft flew by each planet in the 1980s. These pioneering observations were, however, limited in time and sampled specific solar wind/magnetosphere configurations, which significantly vary at various timescales down to a fraction of a planetary rotation. Since then, despite repeated Earth-based observations at similar and other wavelengths, only the Uranian UV aurorae have been re-observed at scarce occasions by the Hubble Space Telescope. These observations revealed auroral features radically different from those seen by Voyager 2, diagnosing yet another solar wind/magnetosphere configuration. Perspectives for the in-depth study of the Uranian and Neptunian auroral processes, with implications for exoplanets, include follow-up remote Earth-based observations and future orbital exploration of one or both ice giant planetary systems. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.