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Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

A Corrigendum to this article was published on 03 October 2017

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Abstract

The Venusian atmosphere is in a state of superrotation where prevailing westward winds move much faster than the planet’s rotation. Venus is covered with thick clouds that extend from about 45 to 70 km altitude, but thermal radiation emitted from the lower atmosphere and the surface on the planet’s nightside escapes to space at narrow spectral windows of the near-infrared. The radiation can be used to estimate winds by tracking the silhouettes of clouds in the lower and middle cloud regions below about 57 km in altitude. Estimates of wind speeds have ranged from 50 to 70 m s−1 at low to mid-latitudes, either nearly constant across latitudes or with winds peaking at mid-latitudes. Here we report the detection of winds at low latitude exceeding 80 m s−1 using IR2 camera images from the Akatsuki orbiter taken during July and August 2016. The angular speed around the planetary rotation axis peaks near the equator, which we suggest is consistent with an equatorial jet, a feature that has not been observed previously in the Venusian atmosphere. The mechanism producing the jet remains unclear. Our observations reveal variability in the zonal flow in the lower and middle cloud region that may provide clues to the dynamics of Venus’s atmospheric superrotation.

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Figure 1: High-pass-filtered radiance at 2.26 μm, horizontal velocities and associated trajectories for 11–12 July 2016.
Figure 2: Zonal wind and vorticity in the lower and middle cloud regions in 11–12 July 2016.
Figure 3: Close up to show the movement of radiance ‘holes’ (bright spots) on 11–12 July 2016.
Figure 4: Zonal winds at the cloud top derived from UVI (365 nm) and IR2 (2.02 μm) data for 11 July 2016.
Figure 5: Zonal winds in the lower cloud at various times.

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Change history

  • 29 August 2017

    In the version of this Article originally published, the directional labels on the x-axes of Figures 1a–d and 2a were W (west), when they should have been E (east). In Figures 2b, 4a,b and 5a–c, the x-axes were given as longitude, when they should have been average wind speed. These errors have been corrected in the online version of the Article.

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Acknowledgements

We sincerely thank the numerous people who contributed to create and operate Akatsuki spacecraft. This study is supported by the following grants: JSPS KAKENHI 15K17767, 16H02225 and 16H02231, 16K17816; NASA Grant NNX16AC79G; JAXA’s International Top Young Fellowship (ITYF). All figures and the supplementary movie were created using the GFD-Dennou Club graphic library, DCL.

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Contributions

T.H. developed automated cloud tracking and error evaluation methods, corrected the bore sight of IR2 nightside images, conducted tracking and interpreted the results. S.-y.M. and K.O. contributed cloud tracking programing. T.S., T.M.S., K.-i.S., T.I. and M.N. conducted IR2 observations and contributed to the operation of Akatsuki and observation planning. K.O., T.K., H.K. and M.T. developed the bore-sight correction applied to dayside images, and they also developed geographical mapping. J.P. conducted manual tracking with IR2 data based on independent geographical mapping. T.S., J.P., T.K., S.S.L., M.T. and E.F.Y. helped scientific interpretation and the review of previous studies. S.W., M.Y. and A.Y. conducted UVI observations. E.F.Y. conducted IRTF observations and the tracking with them. K.M. contributed by optical and cloud-physical interpretation.

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Correspondence to Takeshi Horinouchi.

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Horinouchi, T., Murakami, Sy., Satoh, T. et al. Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki. Nature Geosci 10, 646–651 (2017). https://doi.org/10.1038/ngeo3016

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