Infrared Observations and Investigations of Uranus’s Aurorae and Ionosphere

This thesis presents infrared spectral analyses of Uranus’ observations via the molecular ion H3+. The high spectral and spatial resolution of these observations facilitated the first investigations into Uranus's spatially resolved ionospheric temperature, column densities and ion flows. By documenting these parameters, we can identify and study infrared aurorae and gain insight into Uranian dynamics of magnetosphere-ionosphere coupling.

Using Uranus observations taken by Keck-NIRSPEC, projected maps of intensity, total emission, temperature and column density of the H3+ ions were created. This revealed localised emission enhancements driven by higher H3+ column density. Given these were generated by enhanced ionisation, and the emissions co-location with the Q3, AH5 models and prior UV auroral emissions, we determined this enhancement is a detection of Uranus's infrared aurora.

Analysing Uranus observations from Keck-NIRSPEC and IRTF-iSHELL, we investigated the H3+ line-of-sight velocities at mid-to-low latitudes, derived from the Doppler shift of combined Q(1,0-) and Q(3,0-) emission lines. In calculating the temperature and column density of H3+, we confirm the tentative southern aurora observation by Melin et al., (2019). Enhanced super rotations were observed whilst the southern aurora was present, which may be due to the conservation of momentum via magnetosphere-ionosphere coupling.

Building on these temperature measurements, we also investigate if all emitting energy levels of H3+ at Uranus and Neptune are in local thermodynamic equilibrium using models from Moore et al., (2019 and 2020). We identify that H3+ emissions at Uranus are significantly reduced due to the breakdown of LTE at ~ 1000 km, with Neptune showing a reduced breakdown partially driven by differing models. Our findings also suggest vibrational temperatures of H3+ at Uranus should be 11 to 25% hotter than the rotational, contradicting Trafton et al., (1999). This may be explained by proton transferring which transfers vibrational energy from H3+ to ground state H2.