Nightside Winds at the Lower Clouds of Venus with Akatsuki/IR2: Longitudinal, Local Time, and Decadal Variations from Comparison with Previous Measurements

Abstract

We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXA's mission Akatsuki during 2016, complemented with new wind measurements from ground-based observations acquired with TNG/NICS in 2012 and IRTF/SpeX in 2015 and 2017. Zonal and meridional components of the winds were measured from cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26µm filter, with spatial resolutions ranging 10-80 km per pixel and covering from 2016 March 22 to October 31. More than 149,000 wind vectors were obtained with an automatic technique of template matching, and 2,947 wind vectors were inferred with the manual procedure. The meridional profiles for both components of the winds are found to be consistent with results from the Venus Express mission during 2006-2008, although stronger wind variability is found for the zonal component at equatorial latitudes where Akatsuki observations have better viewing geometry than Venus Express. The zonal winds at low latitudes also suggest a zonal variability that could be associated with solar tides or vertically propagating orographic waves. Finally, the combination of our wind measurements from TNG/NICS, IRTF/SpeX and Akatsuki images with previously published and based in data from 1978 to 2017 suggests variations of up to 30 m s −1 in the winds at the lower clouds of the Venus nightside.

Morphology and Dynamics of Venus's Middle Clouds With Akatsuki/IR1

“…Although consistent at higher latitudes, zonal winds from IR1 peak at the equator and differ from the profile of constant zonal wind between the equator and midlatitudes reported in the past (Belton et al, ; Hueso et al, ; Peralta et al, ; Sánchez‐Lavega et al, ). A similar result was obtained for the zonal winds at the nightside lower clouds with IR2 images (Peralta et al, , figure 5A therein), suspected to be originated by the sporadic jets forming at the equator (Horinouchi et al, ). Confirming previous findings from VEx (Hueso et al, ), no clear trend is observed on the meridional winds (Figure b) and the zonal winds seem to lack of a local time dependence (Figure c).…”

“…near‐infrared or NIR; Belton et al, ), IR1 images were selected attending to the spatial resolution, signal‐to‐noise ratio, and phase angle, prioritizing the presence of cloud features easy to track. Uncertainties in the navigation of IR1 images was corrected using an automatic ellipse fitting (Ogohara et al, ), while for the navigation and adjustment of size/position/orientation of the grid of ground‐based images, we used WinJupos (Hahn & Jacquesson, ) or, alternatively, National Aeronautics and Space Administration's SPICE kernels (Acton, ) and the interactive software created by Peralta et al ().…”

“…The spatial resolution of the projections varies from 16–54 km per pixel (IR1) to ∼530–850 km (ground‐based). Wind measurements: Winds were measured using cloud tracking in pairs of images. A semi‐automatic method (Peralta et al, ) was employed, using phase correlation for the template matching between images and the final result being accepted/rejected by a human operator. When the phase correlation was not effective, the classical visual method undertaken by an operator was used (Peralta et al, ; Sánchez‐Lavega et al, ).…”

“…The size of cloud tracers varies from 770 to 4,100 km (in case of images from amateur observations), being selected depending on the spatial resolution, image signal‐to‐noise ratio and contrast of the observed patterns. We assume that zonal speeds do not dramatically depend on the size if the tracers, as shown for the nightside clouds (Peralta et al, , figure 7A therein). The fast sharp discontinuities on the albedo (Figure c) were interpreted as atmospheric waves and their speeds were discarded from this analysis.…”

A Long‐Lived Sharp Disruption on the Lower Clouds of Venus

“…(a) Morphological changes of the cloud discontinuity during 2016 on images from IR1/900‐nm (dayside) and IR2/2.26‐μm (nightside). Full set in Figures S2–S3; (b) example of undulations behind a discontinuity in 15 April 2016; (c) discontinuities apparent on the middle clouds (IR1) but not at the upper clouds sensed with ultraviolet (UVI) and 2.02‐μm images (IR2); (d) past events of disruption (left‐to‐right, top‐to‐bottom): September 1983 (Allen & Crawford, 1984), January–February 1990 (Crisp et al, 1991), December 2005 (Bailey, 2006), September 2006 (VEx/VIRTIS), and July 2012 (Peralta et al, 2018). All images were high‐pass filtered (see section 2.1).…”