Comparative aeronomy of the upper atmosphere of the giant planets
This thesis presents a study of H3 emission from Jupiter, Saturn and Uranus. This emission is associated with both auroral and solar processes, and can be used to trace energy inputs in the upper atmosphere. For Jupiter, a detailed analysis of the H3 emission is performed using a ID self-consistent atmospheric numerical model and the detailed balance formulation for H3 contained within a volume of H2. It is shown that the effects due to departures from local thermodynamic equilibrium (LTE) are significant, reducing the intensity of the observed H3. Evidence of this is shown to be present in published observations of the jovian aurora. Using the non-LTE results, the auroral heating event observed by Stallard et al. 2002 on Jupiter is analysed in terms of energy inputs and outputs. It is shown that the dominant heating source is Joule heating and ion-drag, produced by the increase in ion velocity in the auroral electrojet. However, in the absence of the heating there are not enough heat sinks as to cool down the atmosphere quickly, indicative that the energy is re-distributed mechanically via thermally driven winds. Using data from the United Kingdom Infrared Telescope (UKIRT), the first reliable auroral H3" temperature of Saturn is determined: T = 400 50 Kelvin with a column- integrated H3" density of N = 4.6 x 1016 m-2. The variations in column density within the dataset are large, indicating large variations in the precipitation flux, while the temperature remains fairly constant. On Uranus, infrared H3 spectra covering a decade is analysed in an attempt to explain the observed variability. This forms the most comprehensive long-term study of H3 emission to date. The variability is found to be complicated, with an auroral component of the total Hjj" emission being of similar magnitude as the EUV-produced component. The auroral component appears to be controlled by geometry.