Dynamics of Saturn's Polar Regions

Abstract: We analyze data retrieved by the Imaging Science System onboard the Cassini spacecraft to study the horizontal velocity and vorticity fields of Saturn's Polar Regions (latitudes 60-90$^\circ$N in June-December 2013 and 60-90$^\circ$S in October 2006 and July-December 2008), including the Northern region where the hexagonal wave is prominent. With the aid of an automated two dimensional correlation algorithm we determine two-dimensional maps of zonal and meridional winds, and deduce vorticity maps. We extract zonal averages of zonal winds, providing wind profiles that reach latitudes as high 89.5$^\circ$ in the south and 89.9$^\circ$ in the north. Wind measurements cover the intense polar cyclonic vortices that reach similar peak velocities of 150 ms-1 at 88.5$^\circ$. The hexagonal wave lies in the core of an intense eastward jet at planetocentric latitude 75.8$^\circ$N with motions that become non-zonal at the hexagonal feature. In the south hemisphere the peak of the eastward jet is located at planetocentric latitude 70.4$^\circ$S. A large anticyclone (the South Polar Spot, SPS), similar to the North Polar Spot (NPS) observed at the Voyager times (1980-81), has been observed in images from April 2008 to January 2009 in the South Polar Region at latitude -66.1$^\circ$ close to the eastward jet. The SPS does not apparently excite a wave on the jet. We analyze the stability of the zonal jets, finding potential instabilities at the flanks of the eastward jets around 70$^\circ$ and we measure the eddy wind components, suggesting momentum transfer from eddy motion to the westward jets closer to the poles.