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

We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXA's mission Akatsuki during 2016, complemented by new wind measurements from ground-based observations acquired with the TNG/Near Infrared Camera Spectrometer (NICS) in 2012 and IRTF/SpeX in 2015 and 2017. The zonal and meridional components of the winds were measured via cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26 mu m filter, with spatial resolutions ranging from 10 to 80 km per pixel and covering 2016 March 22 to October 31. More than 149,000 wind vectors were obtained using an automatic template-matching technique, and 2947 wind vectors were inferred using a 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 data from 1978 to 2017 suggests variations of up to 30 m s(-1) in the winds at the nightside lower clouds of Venus.