Abstract
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'.
Highlights
Uranus and Neptune possess multipolar magnetic fields, indicating complex planetary interiors, with comparable dipole amplitudes
The diversity of ice giant radio emissions is reviewed at length in [11,12,13]. We focus on their prominent auroral component, observed at kilometric wavelengths within 15 − 900 kHz and 10 − 1300 kHz (Figure 2) and termed Uranian and Neptunian Kilometric Radiations (UKR and NKR), respectively
UV and NIR Earth-based facilities proved to be capable of detecting emissions from Uranus, and are worthy to pursue with current (HST, NIR ground-based telescopes) and future (JWST, LUVOIR) observatories, while X-ray observations remain exploratory so far
Summary
Uranus and Neptune possess multipolar magnetic fields, indicating complex planetary interiors (see Helled & Forteney and Soderlund & Stanley, this issue), with comparable dipole amplitudes These emissions, radiated above and within the atmosphere by energetic electrons gyrating along high latitude magnetic field lines, appeared more complex than expected They displayed features reminiscent of the well known aurorae of the Earth, Jupiter or Saturn together with components unique to Uranus and Neptune, with implications for the search for exoplanets. They were used to determine the rotation rate of their interior and to probe the magnetic field topology, magnetospheric dynamics and acceleration processes. Only the Uranian UV aurorae were successfully re-observed at a couple of occasions by the Hubble Space Telescope (HST) past equinox It revealed auroral features radically different from those monitored by Voyager 2, diagnosing yet another solar wind/magnetosphere configuration. This article aims at briefly reviewing our current knowledge of Uranian and Neptunian auroral emissions at radio (section 2) and UV wavelengths (section 3) and their implications on magnetospheric dynamics, the search for auroral counterparts in the NIR and X-ray ranges (section 4), before ending with perspectives for the in-depth study of ice giant auroral processes (section 5)
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