Infrared Observations of Saturn’s Aurorae, Ionosphere and Thermosphere

This thesis presents analyses of infrared spectral observations of Saturn’s northern H+3 aurorae. These very high spectrally resolved data facilitated investigations of ion flows in the planet’s ionosphere and the thermospheric temperature, thus elucidating the driver of planetary andmagnetospheric periodicities witnessed at Saturn.

Using observations taken in May 2013 with the VLT-CRIRES instrument – previously available at the European Southern Observatory’s Very Large Telescope – an investigation of the H+3 auroral emission intensity, ion line-of-sight velocity (derived from Doppler shifts of a H+3 emission line), and rotational temperature directly along the northern pole (perpendicular to the axis of planetary rotation) was carried out. Evidence of a dawn-enhanced auroral emission with an average temperature of 399 (±49) K, including a localised dark region within the emission co-located with an ∼1 km s−1 noon-to-midnight (and vice versa) ion flow in the H+3 line-of-sight velocity, tantalisingly hinted at the presence of an ionospheric polar vortex. 

Mapped observations from June, July and August 2017 of Saturn’s northern aurorae taken using the Keck-NIRSPEC instrument were used to further investigate H+3 ion flows after initially grouping individual spectra into quadrants of northern planetary magnetic phase. Comparing the H+3 ion line-of-sight velocities from opposing phase quadrants led to a direct detection of the ionospheric ‘twin-vortex’ mechanism, thus confirming that the upper atmosphere drives these current systems.

The Keck-NIRSPEC data were also used to compute the H+3 rotational temperature after grouping them into the same rotational phase quadrants as for the ion line-ofsight velocities. Despite the relatively poor signal of the H+3 R(2,2−) emission line, the temperature profiles did not reveal any evidence of a localised thermal ‘hot spot’. This suggests that the driving mechanism for the planetary period currents may not be situated within the neutral thermosphere as had been hypothesised.