Saturn's north polar vortex structure extracted from cloud images by the optical flow method

• Published in: Journal of Geophysical Research: Planets
• Main Authors: Tianshu, Liu, Sayanagi, Kunio M., Brueshaber, Shawn R., Blalock, John J., Ingersoll, Andrew P., Dyudina, Ulyana A., Ewald, Shawn P.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2019
• Subjects: Saturn; north polar vortex; flow structure; optical flow; cloud tracking; velocity; North Pole
• Online Access: https://doi.org/10.1029/2019JE005974

The paper presents velocity fields with ~3â€km spatial resolution of Saturn's north polar vortex (NPV) retrieved using the optical flow method from a sequence of polarâ€projected cloud images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft. The fields of the velocity magnitude, velocity variation, relative vorticity, divergence, and second invariant are determined to characterize the flow structures of the inner core of the NPV. The mean zonal and mean meridional velocity profiles of the NPV are compared with previous measurements. We also describe the relevant details of application of the optical flow method to planetary cloudâ€tracking wind measurements. The mean zonal velocity profile is consistent with the previous measurements using correlation image velocimetry methods. The small but significant meridional velocity corresponds to outwardly spiraling streams observed in the region near the north pole (NP). The concentrated vorticity and second invariant within 1° planetographic latitude of the NP indicate strong rotational motion of the fluid. An analysis is presented to explore a possible physical origin of the observed spiraling node at the NP. © 2019 American Geophysical Union. Received 25 MAR 2019; Accepted 25 SEP 2019; Accepted article online 15 OCT 2019; Published online 26 NOV 2019. K.M.S. was supported by NASA Cassini Data Analysis Program Grant NNX15AD33G. S.R.B. was supported by NASA NESSF Grant NNX16AP44H. The data used to generate the figures in this paper are available in "Saturn_NPV_Velocity_Data" at Harvard Dataverse. The link is https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi%3A10.7910%2FDVN%2FOPLQCL Published - Liu_et_al-2019-Journal_of_Geophysical_Research__Planets.pdf