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Aurorally Driven Supersonic Gravity Waves in Saturn's Atmosphere
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Aurorally Driven Supersonic Gravity Waves in Saturn's Atmosphere
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Aurorally Driven Supersonic Gravity Waves in Saturn's Atmosphere
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Journal Article
Aurorally Driven Supersonic Gravity Waves in Saturn's Atmosphere
2024
Overview
Simulations with the Saturn Thermosphere‐Ionosphere General Circulation Model have revealed global‐scale gravity waves in Saturn's upper atmosphere that have not been observed or predicted before. They are forced by diurnally varying zonally non‐uniform distributions of Joule heating and ion drag at the high‐latitude auroral regions in both hemispheres and propagate outward from the source region. The supersonic zonal phase speed imposed by Saturn's rotation and the subsonic meridional phase velocity of about 800 m s−1${\\mathrm{s}}^{-1}$form a distinct spiral wave structure. They exhibit features of gravity waves: vertical phase progression opposite to the propagation of wave energy and long vertical wavelengths consistent with the dispersion relation for gravity waves. The amplitudes of the perturbations grow with height, reaching 6%–10% for relative temperature variations and up to 350 m s−1${\\mathrm{s}}^{-1}$for the meridional velocity perturbations. The main effect of these waves is to accelerate the retrograde westerly jets. Plain Language Summary Gravity waves are common in all convectively stable atmospheres. They are caused by a variety of sources and have horizontal extents ranging from a few kilometers to scales comparable to the radius of the planet. In simulations of the Saturnian upper atmosphere, we found a new form of gravity wave that has not been observed or predicted before. The waves are excited at 900–1,000 km above the 1 bar pressure level near the auroral ovals in both hemispheres. They are caused by the changing heating and drag in the auroral regions as Saturn rotates throughout the day. Saturn's large size and fast rotation make the waves move around the planet faster than the speed of sound in Saturn's atmosphere. They spread out from their source, creating a spiral pattern that covers a large part of the planet. We studied these waves and how they affect the global circulation. Unlike gravity waves on Earth and similar planets, which mostly slow down the surrounding winds, these waves in Saturn's upper atmosphere mainly speed up the strong westward jets. Key Points General circulation modeling reveals large‐scale gravity waves in the upper atmosphere of Saturn that have never been reported before Waves are forced by Joule heating and ion drag at high‐latitude auroral regions, propagate outward and form a global spiral structure The net dynamical effect of these zonally supersonic and meridionally subsonic waves is to accelerate the westward retrograde jets
