A laboratory model of Saturn’s North Polar Hexagon

Emergence of polar-jet polygons from jet instabilities in a Saturn model

“…The eddies that form on the flanks of the easward jet in this simulation are stronger than the eddies that form in the simulations initialized with simple Gaussian jets, and also stronger than those found by Godfrey's in the anticyclonic shear zone outside of Saturn's Hexagon. In this regard, our simulations are comparable to some laboratory experiments that show that the stability and coherence of the polygonal wave pattern is a manifestation of the vortex street structure (Niino and Misawa, 1984;Marcus and Lee, 1998;Barbosa-Aguiar et al, 2010).…”

“…Several laboratory experiments have shown how vortex streets can shape polygonal flows in fluids in a rotating tank for a wide range of parameters and as a result of different kinds of forcing (Sommeria et al, 1989;Vatistas, 1990;Vatistas et al, 1994;Marcus and Lee, 1998;Jansson et al, 2006). More recently, Barbosa-Aguiar et al (2010) have shown how the zonal flow around the latitude of the north polar Hexagon is unstable according to linear barotropic theory. They also have shown how polygonal structures, corresponding to wave modes caused by the nonlinear equilibration of barotropically unstable zonal jets, can appear in laboratory experiments of flows in rotating tanks.…”

A δ-plane simulation of anticyclones perturbing circumpolar flows to form a transient north polar hexagon

“…Numerical simulations using the Explicit Planetary Isentropic-Coordinate (EPIC) model found that a stable hexagonal structure can result without forcing (MoralesJuberias et al 2011). The EPIC simulations are in agreement with the experimental findings of Barbosa-Aguiar et al (2010) and the resulting hexagonal structure arises due to the formation of what is termed a 'vortex street', where there is an alignment of opposing vortices such that there exists a meandering jet separating them. In the case of the Saturn hexagon simulations, there are six anticyclones slightly south of the six cyclones.…”

“…This type of experiment has given rise to alternate explanations of the origin of the North Polar Hexagon. In particular, laboratory experiments with fluids in rotating tanks in conjunction with a barotropic linear instability analysis of Saturn's zonal wind profile were used to infer that Saturn's Hexagon is the result of equilibrated wavemodes of the barotropic instability, with mode six being the preferred state on Saturn due to the location and strength of the eastward circumpolar jet (Barbosa-Aguiar et al 2010). Numerical simulations using the Explicit Planetary Isentropic-Coordinate (EPIC) model found that a stable hexagonal structure can result without forcing (MoralesJuberias et al 2011).…”

“…Although the experimental (Barbosa-Aguiar et al 2010) and numerical (Morales-Juberias et al 2011) results produce hexagonal features as a bi-product of a 'vortex street', there remains doubt over the legitimacy of such a model producing the actual North Polar Hexagon on Saturn, due to the debate over the existence of large vortices being observed in the vicinity of the Hexagon region of Saturn (Morales-Juberias et al 2011;Sanchez-Lavega et al 2014). A visible 'vortex street' may not be apparent in the Cassini images (Sanchez-Lavega et al 2014), but it has been shown previously by analysing the original Voyager images that there was a visible anticyclone and several other non-visible anticyclonic regions aligned centrally along the southern side of the Hexagon edges (Godfrey 1988), but the lack of cyclonic regions to the north exclude it from being the result of a 'vortex street'.…”