We present a study of Saturn’s H3+ northern auroral emission using data taken on 19 May 2013 with the Very Large Telescope’s long-slit spectrometer Cryogenic Infrared Echelle Spectrograph (VLT-CRIRES) situated at Paranal, Chile. Adaptive optics, combined with the high spectral resolution of VLT-CRIRES (~100,000), mean that this dataset offers an unprecedented spatially and spectrally resolved ground-based view of Saturn's infrared aurora. We have used discrete H3+ emission lines to derive dawn-to-dusk auroral intensity, ion line-of-sight velocity, and thermospheric temperature profiles.

Our data reveal a dawn-enhanced auroral intensity with an average auroral temperature of 361 K and evidence for a localised dark polar region in the aurora. This dark feature is at the same location as a strong noon-midnight flow in the ion velocity on the scale of ~1.2 km per second – far exceeding other ion flow velocities nearby inside the polar cap – and resembles an ionospheric polar vortex. The thermospheric temperature profile reveals a subtle and previously undetected gradient which increases across the polar cap going from dawn to dusk. We also find that a temperature hotspot of 379 K drives a region of emission near the pole, corresponding to a location where, unlike at Jupiter and Uranus, H3+ is failing to cool the thermosphere.

The findings of this work will aid ongoing investigations into the processes driving the energy mechanisms in Saturn’s upper atmosphere and also inform models of the coupling between Saturn’s upper atmospheric layers and its magnetic field.