Evidence of a Polar Cyclone on Uranus From VLA Observations

Abstract

Uranus' obliquity affords unique opportunities for polar observing, which can contribute to our general understanding of polar circulation processes on giant planets. Our current understanding of Uranus' polar circulation (we define the pole as >60° latitude) suggests net subsidence as the characteristic feature of vertical motion. This pattern is inferred from emission and reflection contrasts in multi-wavelength observations (de Pater et al., 1989(de Pater et al., , 1991Hofstadter & Butler, 2003;Roman et al., 2020;Sromovsky et al., 2019), which suggest depletion of absorbing gases at the poles down to 10s of bars. Zonal wind profiles of polar latitudes, estimated nominally at 1.5 bar, have been most recently updated by Karkoschka (2015) at southern latitudes based on Voyager 2 images and Sromovsky et al. ( 2015) at northern latitudes based on Keck observations. Mean zonal wind speeds decay monotonically from a maximum of 250 m/s near 60° to near-zero at 90°, and there appears to be significant asymmetry between the northern and southern pole zonal wind profiles (Karkoschka, 1998;Sromovsky & Fry, 2005;. This asymmetry, as well as analyses of Voyager gravity measurements, suggests that zonal wind speeds are vertically confined globally and should decay with altitude in the upper troposphere (e.g., see Fletcher et al. ( 2020) for a recent review). As new observations and analysis techniques push limitations of spatial resolution and sensitivity, new features have emerged in images of Uranus that complicate this broader picture. For example, complex polar features have been observed at the CH 4 weather level, including a multitude of compact bright features in the northern hemisphere suggestive of small-scale vertical convection acting against net subsidence (Sromovsky et al., 2012. At many wavelengths, this is the first solstice for which ground-based and near-Earth observatories are sufficiently sensitive to resolve fine details in polar thermal emission. This is certainly true for microwave observations with the VLA, which underwent an order of magnitude bandwidth upgrade in 2012 that significantly improved the achievable sensitivity. Unlike mid-infrared, near-infrared, and visible observations, which have informed our understanding of Uranus' circulation at and above the H 2 S and CH 4 cloud levels, microwave observations are sensitive to thermal emission from deeper in the troposphere (primarily 10-50 bar at the poles). Post-upgrade VLA observations of Uranus are now sufficiently sensitive to resolve faint zonal banding patterns at lower latitudes at the 10 bar level. Observations of Uranus in 2015 described by Molter et al. (2021), specifically the Ku band (2 cm) image, provided evidence of latitudinal structure in tropospheric thermal emission at Uranus'

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