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.