Numerical simulations of atmospheric waves in Saturn's upper atmosphere

In the atmospheres of the Solar System planets, several kinds of atmospheric waves can be found. They are an important atmospheric phenomenon as, besides disturbing the atmosphere, they can also alter the mean structure and affect the general circulation [1]. Therefore, understanding their effects on the atmospheric dynamics is of paramount importance.

The thermospheres of the Giant Planets are hotter than what would be expected if solar heating were considered the only energy source. Historically, the few existing numerical models have had difficulties to reproduce the observed temperature structure at mid and low latitudes. It was long thought that waves propagating vertically might play an important role in determining the temperature by either heating the thermosphere [2, 3] or by weakening the intense high-latitude westward jets allowing the meridional transport of energy trapped in the polar regions [4, 5]. Recently, the detection of gravity waves in Saturn's thermosphere has been reported using data from Cassini INMS and UVIS occultation data [4, 5, 6].

The Saturn Thermosphere-Ionosphere Model (STIM) is the only 3D general circulation model published so far for Saturn's upper atmosphere [7, 8]. The model couples dynamically and chemically the thermosphere and the ionosphere. In this work, we have used the STIM model to perform high resolution numerical simulations of Saturn's thermosphere. They show the development of supersonic spiral gravity waves in the northern and southern auroral regions driven by the high latitude forcing produced by Joule heating and ion drag. We analyze the morphology and properties of these waves, their propagation and the forcing they impose on the mean flow. We also show simulations with forcing introduced at the bottom of the model to explore waves that can propagate through the thermosphere and their potential impact on the circulation.

 

References:

[1] Vallis. Atmospheric and Ocean Fluid Dynamics. Cambridge University Press, Cambridge, UK, 2006.

[2] Young et al. Gravity waves in Jupiter's thermosphere. Science, 276, 1997.

[3] O'Donoghue et al. Heating of Jupiter's upper atmosphere above the Great Red Spot. Nature, 536, 2016.

[4] Müller-Wodarg et al. Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere. Geophysical Research Letters, 46, 2019.

[5] Brown et al. Evidence for gravity waves in the thermosphere of Saturn and implications for global circulation. Geophysical Research Letters, 49, 2022.

[6] Brown et al. A pole-to-pole pressure-temperature map of Saturn's thermosphere from Cassini Grand Finale data. Nature Astronomy, 4, 2020.

[7] Müller-Wodarg et al. A global circulation model of Saturn's thermosphere. Icarus, 180, 2006.

[8] Müller-Wodarg et al. Magnetosphere-atmosphere coupling at Saturn: 1 – Response of thermosphere and ionosphere to steady state polar forcing. Icarus, 221, 2012.