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Vertical Wind Shear in Neptune's Upper Atmosphere Explained with a Modi ed Thermal Wind Equation

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journal contribution
posted on 23.07.2018, 15:18 by Joshua Tollefson, Imke de Pater, Philip S. Marcus, Statia Luszcz-Cook, Lawrence A. Sromovsky, Patrick M. Fry, Leigh N Fletcher, Michael H. Wong
We present observations of Neptune taken in H-(1.4–1.8 µm) and K’-(2.0–2.4 µm) bands on the nights of July 3, 2013 and August 20, 2014 from the 10-m W.M. Keck II Telescope using NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of ∼100 bright atmospheric features over a 4–5 h window on each night to derive zonal velocities and wind profiles. Our results deviate from the smooth Voyager zonal wind profile from Sromovsky et al. (1993), often by 100–200 m/s, and often by 3–10 times their estimated uncertainties. Besides what appears to be a random dispersion, probably due to a mix of unaccounted for measurement errors, eddy motions, vertical wind shear, and wave-generated features that do not follow the mass flow, there is also a systematic deviation that is wavelength dependent. The H-band profile is best described with a 73–106 m/s shift towards the east for a retrograde flow (i.e., a lessening of the retrograding velocities) from the Voyager profile at the equator. The K’-band profile is consistent with Voyager on both nights. Comparing H and K’ contribution functions and K’/H intensities suggests equatorial H-band features are, on average, deeper than K’-band features. The H-band equatorial features also have greater eastward (less negative) velocities than K’-band features. Differences in zonal wind speed with depth at constant latitude and time imply vertical wind shear. Assuming the average variations in the zonal wind profiles result from wind shear over 3–5 scale heights, we predict vertical wind shears between −1.0 and −2.2 m/(s km) at the equator (increasing with height). The standard thermal wind equation and meridional thermal profile for Neptune given by Voyager/IRIS spectra predict wind shear of the wrong sign relative to the observations. We consider two effects that reconcile this inconsistency. First, we calculate the meridional temperature gradients at pressures outside the Voyager/IRIS narrow sensitivity window required to match our predicted wind shears. Second, we generalize to a thermal wind equation that considers global methane variations and re-derive the temperature structure needed to match the observed wind shear. If methane is uniformly distributed or weakly varying, the equator must be 2–15 K cooler than the mid latitudes below 1 bar. If methane is strongly varying, the equator can be 2–3 K warmer than the mid latitudes below 1 bar, qualitatively consistent with observed temperature contrasts. These findings may imply a stacked-celled circulation pattern in Neptune’s troposphere and lower stratosphere.

Funding

The data presented were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. This work has been supported in part by the National Science Foundation, NSF Grant AST-0908575 and AST-1615004 to UC Berkeley and by NASA Headquarters: under the NASA Earth and Space Science Fellowship program Grant NNX16AP12H to UC Berkeley, and Grant NNX23AG56G under NASA Planetary Atmospheres to UC Berkeley. LNF was supported by a Royal Society Research Fellowship and European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement No. 723890) at the University of Leicester. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to conduct observations from this mountain.

History

Citation

Icarus, 2018, 311, pp. 317-339

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

AM (Accepted Manuscript)

Published in

Icarus

Publisher

Elsevier for Academic Press

issn

0019-1035

Acceptance date

11/04/2018

Copyright date

2018

Available date

19/04/2019

Publisher version

https://www.sciencedirect.com/science/article/pii/S001910351630210X?via=ihub

Notes

Supplementary material associated with this article can be found, in the online version, at 10.1016/j.icarus.2018.04.009.;The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.

Language

en

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