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Venus' cloud top wind study: Coordinated Akatsuki/UVI with cloud tracking and TNG/HARPS-N with Doppler velocimetry observations

R. Gonçalves, P. Machado, T. Widemann, J. Peralta, S. Watanabe, A. Yamazaki, T. Satoh, M. Takagi, K. Ogohara, Y. J. Lee, J. Silva

Abstract
We present new wind velocities and its variability at Venus cloud-tops (≈70 km) based on coordinated observations with two complementary techniques: (1) space-based cloud-tracking using JAXA's Akatsuki/UVI imaging and (2) ground-based Doppler velocimetry using the TNG/HARPS-N (Telescopio Nazionale Galileo). This was the first use of HARPS-N to observe a Solar system planetary atmosphere. The measurement of both zonal and meridional wind velocities and the constraints of temporal and spatial wind variability at cloud top is essential to our understanding of the mechanisms which yield and maintain the dynamics of Venus' atmospheric superrotational circulation.

From space, clouds features were tracked on images obtained by the Akatsuki instrument (UVI) operating in the ultraviolet range (365 nm filter), acquired in orbit #39, between 26 and 31 of January 2017. From the ground, data acquired on 28–29 January 2017 using TNG/HARPS-N, Doppler velocimetry measurements are based on high-resolution spectra (resolution of ∼115,000) of Fraunhofer lines in the visible range (383–690 nm) using incoming solar radiation scattered by cloud top particles in the observer's direction (Widemann et al., 2008, Machado et al., 2014, Machado et al., 2017).

The analysis and results of this new coordinated dataset indicate (1) cross-validation and complementarity of cloud-tracking and Doppler velocimetry measurements; (2) a significant North-South asymmetry of zonal wind circulation, of the order of 10 ms−1 higher in the South hemisphere, in Akatsuki/UVI observations (in accordance with previous detection by Horinouchi et al. (2018)); (3) we present a high precision cloud-top meridional wind flow, obtained with HAPRS-N; a poleward meridional flow, symmetrical with respect to equator, clearly measured in both hemispheres; it peaks around 40° N and S with an amplitude of = 30±5 ms−1; (4) spatial and temporal variability of Akatsuki's zonal and meridional wind flow is also observed, with a significant increase of wind flow near evening terminator.

Keywords
Venus; Atmosphere; Dynamics; Akatsuki; Spectroscopy; Cloud tracking; Doppler velocimetry

Icarus
Volume 335, Number 1134, Page 335
2020 January

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia