MIST
Magnetosphere, Ionosphere and Solar-Terrestrial
Exploring Key Characteristics in Saturn’s Infrared Auroral Emissions Using VLT-CRIRES: H3+ Intensities, Ion Line-of-Sight Velocities, and Rotational Temperatures
by Nahid Chowdhury (University of Leicester)
Saturn’s aurorae are generated by interactions between high-energy charged particles and neutral atoms in the upper atmosphere. Infrared observations of auroral emissions make use of H3+ – a dominant hydrogen ion in Saturn’s ionosphere – that acts as a tracer of energy injected into the ionosphere.
We analysed observations taken in May 2013 of Saturn’s northern infrared auroral emissions with the Very Large Telescope in Chile using the CRIRES instrument. The use of adaptive optics, combined with the high spectral resolution of VLT-CRIRES (100,000), meant that this dataset offered an unprecedented spatially and spectrally resolved ground-based view of Saturn's infrared aurora. Using discrete H3+ emission lines, we derived dawn-to-dusk auroral emission intensity, ion line-of-sight velocity, and thermospheric temperature profiles, allowing us to probe the physical properties of Saturn’s polar atmosphere.
Our analysis showed an enhancement in the dawn-side auroral emission intensity, a common feature that is known to be linked with solar-wind compressions in the kronian magnetosphere, and the presence of a localised dark region in the aurora very close to the pole. The ion line-of-sight velocity profile revealed previously unknown smaller-scale structures in the ion flows. In particular, the ion flows near the centre of the pole (at position B in Figure 1) could be consistent with the behaviour of a relatively small ionospheric polar vortex whereby the ions are interrupting the general dawn-to-dusk trend in movement to instead adopt a very sharp shearing motion of ions first toward midnight and then almost immediately back toward noon. Our thermospheric temperature derivations also reveal a very subtle temperature gradient that increases from 350 K on the dawn-side of the pole to 389 K on the dusk-side.
This work has bought to light complex features in the behaviour of H3+ ions in Saturn’s upper atmosphere for the first time and highlights the need for additional analyses of two-dimensional scanned maps of Saturn’s auroral regions with a view to addressing some of the major outstanding questions surrounding Saturn’s thermosphere-ionosphere-magnetosphere interaction.
For more information, please see the paper below:
, , , & ( 2019). Exploring key characteristics in Saturn's infrared auroral emissions using VLT‐CRIRES: H3+intensities, ion line‐of‐sight velocities, and rotational temperatures. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL083250.
Figure 1: The ion line-of-sight velocity and auroral emission intensity profiles are plotted as a function of co-latitude on the planet. Evidence for ion flows possibly consistent with the behaviour of an intriguing ionospheric polar vortex is adjacent to the area marked by the letter B, between approximately 0⁰ and 5⁰ co-latitude on the dawn-side of Saturn’s northern pole.