Simulations of gravity waves in Saturn's thermosphere.

Several types of atmospheric waves can be found in the atmospheres of the Solar System planets. They are an important atmospheric phenomenon because they can modify the mean structure of the atmosphere and affect the general circulation [1]. Gravity waves are a common type of atmospheric wave. They can have different origins and cover a large range of spatial scales. Gravity waves are able to transport energy and momentum through different atmospheric layers, thus indicating the great importance of understanding their effects on the atmospheric dynamics.

Observations of temperatures of the Giant Planets have shown that their upper atmospheres (thermospheres) are hundreds of Kelvins hotter than would be expected from solar heating alone [2]. The existing numerical models have had difficulties to reproduce the observed temperature structures, particularly at mid and low latitudes. It has been long thought that waves propagating vertically from below play an important role in shaping the thermal structure of thermospheres. One of the mechanisms proposed to heat thermospheres is the heating produced by dissipating waves [3, 4], but this effect seems to be at least a factor of two lower than needed on Saturn [2]. Another mechanism is the weakening of the intense high-latitude westward jets, thereby allowing the meridional transport of energy trapped in the polar regions [5, 6]. Recently, the detection of gravity waves in Saturn's thermosphere has been reported using data from Cassini INMS and UVIS occultation data [5, 6, 7].

The Saturn Thermosphere-Ionosphere Model (STIM) is a 3D general circulation model used to study the dynamics, energy balance and gas structure of Saturn's upper atmosphere under the external influences of solar radiation and magnetospheric forcing [8, 9]. The model couples dynamically and chemically the thermosphere and the ionosphere, including the drag produced by the collisions between ions and neutral species, and Joule heating. In this work we show high resolution numerical simulations of Saturn's upper atmosphere performed using the STIM model to explore the effects of gravity waves in the circulation of Saturn's thermosphere. We force the bottom of the model (near 3 Pa) to explore the propagation of waves resolved by the model through the thermosphere, their characteristics and impact on the circulation. We also explore the effects of smaller scale gravity waves by adapting the gravity wave drag parameterization of [10] to Saturn.

 

References:

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

[2] Strobel et al. Saturn's Variable Thermosphere. Saturn in the 21^st Century. Cambridge University Press, Cambridge, UK, 2019.

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

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

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

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

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

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

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

[10] Yiğit et al. Parameterization of the effects of vertically propagating gravity waves for thermosphere general circulation models: Sensitivity study. Journal of Geophysical Research, 113, 2008.