Evidence for Gravity Waves in the Thermosphere of Saturn and Implications for Global Circulation.

Gravity wave (GW) signatures have been derived from temperature profiles observed by Cassini/Ultraviolet Imaging Spectrograph in the Saturnian thermosphere during the Grand Finale campaign. They demonstrate upward propagation of GW packets, their saturation, and breaking. We determined wave amplitudes, potential energy, and momentum fluxes and estimated the associated wave drag imposed by dissipating harmonics on the ambient flow. The data set of 18 profiles covers the middle and high latitudes of both hemispheres, which allows for exploring the global impact of waves. The diagnostics based on the Transformed Eulerian Mean and modified geostrophy approach reveal that the GW drag induces an equatorward flow in both hemispheres, facilitating transport of heat away from the auroral zones and redistributing energy across latitudes. Like all the outer planets, Saturn's thermosphere is hundreds of degrees hotter than what follows from radiative balance and these results help to explain the observed temperatures at all latitudes. Plain Language Summary: Gravity waves are small‐scale fluctuations of air density, temperature, and other atmospheric variables, which are dynamically important in the upper atmospheres. During Cassini's Grand Finale, observations with the Ultraviolet Imaging Spectrograph delivered a set of density profiles in the Saturnian thermosphere with a resolution sufficient to detect such waves. We derived various characteristics and estimated the forcing imposed by dissipating waves on the global circulation. It turned out that this "gravity wave drag" causes enhanced flow toward the equator in both hemispheres, transporting heat away from the auroral sources in high latitudes and distributing it over Saturn's thermosphere. Previous studies suggested that such redistribution would be prevented by fast westward jets (the "energy crisis"). We show that the waves observed on Saturn can successfully help winds to overcome this barrier. A similar mechanism can exist on other outer planets, whose thermospheres are also much hotter than what follows from radiative balance. Key Points: We present evidence for the omnipresence of gravity waves in the Saturnian thermosphere from Cassini/Ultraviolet Imaging Spectrograph Grand Finale measurementsWe determine wave amplitudes, energy, momentum fluxes, and forcing imposed on the mean flow along with their spatial distributionsGravity wave drag enhances pole‐to‐equator flow in both hemispheres, which helps to redistribute energy across latitudes [ABSTRACT FROM AUTHOR]