The upper atmosphere, ionosphere, and aurora of Uranus and Neptune

Uranus is perhaps the strangest planet in our solar system, with a rotational axis aligned with the ecliptic, an offset magnetic field that places the magnetic poles close the equator, and almost no internal heat. Neptune has as similar magnetic configuration, but has a large internal heat source, generating the fastest winds seen anywhere in our solar system. An important ion in the upper atmosphere of the giant planets is the molecular ion H3+, created via the ionisation of molecular hydrogen. Via the analysis of the H3+ spectrum, the temperature of the upper atmosphere and density of the ionosphere can be remotely determined using near-infrared spectrographs on ground-based telescopes. Since the detection of H3+ emissions from Uranus in 1992, the planet has been routinely observed. Here, we discuss the mysterious long-term cooling discovered in the upper atmosphere of Uranus, and the scant auroral observations, providing clues as to how the planet interacts with its space environment. Surprisingly, emissions from H3+ remains undetected from the other ice giant, Neptune, and the potential reasons for this are explored.