Sayanagi, K. M., Muro, G. D., Ewald, S. P., Ingersoll, A. P. (2010). "Cassini ISS Analysis of Meandering Jets on Saturn: Ribbon and Hexagon" American Astronomical Society, DPS meeting #42, #41.02.


Abstract
The jetstreams on Saturn at 47N and 77N planetographic latitude are notable in exhibiting wavy cloud morphologies that have been called the Ribbon and the Hexagon, respectively. Recent laboratory and numerical experiments identified several distinct scenarios that can lead to meandering zonal jets. For the Ribbon, the 3D numerical model of Sayanagi et al. (in print, JAS) predicts that the jet is in a dynamical regime where its path continuously shifts over short timescales. For the Hexagon, laboratory experiments by Barbosa Aguiar et al. (2010, Icarus) and 3D numerical simulations by Morales-Juberias et al. (under review, Icarus) demonstrate that a barotropically unstable zonal jet can equilibrate into a steady vortex street. The dominant wavenumber depends on jet parameters, and wavenumber 6 is a possible outcome. We also present a scenario, based on the shallow water equations, in which a steady jet meanders around a sharp potential vorticity gradient without an accompanying vortex street. We test these theories against observation. The Ribbon emerged from the ring shadows in mid-2007. Today, we find cloud morphologies substantially different from those of the Voyager era (Godfrey and Moore, 1986 Icarus), although the 47N jet exhibits similar wave activity. We use the temporal dynamics of the waves to test the predictions of Sayanagi et al. The Hexagon has been illuminated by sunlight since late 2008 and we have confirmed that its structure retains the characteristics found by Voyager (Godfrey, 1988 Icarus) and Cassini VIMS (Baines et al., 2009 P&SS). We measure the vorticity field in the Hexagon region to determine whether the structure is consistent with a vortex street. We also compare the zonal wind profiles of the regions against past measurements.

Our study is supported by the Cassini project.