Unbiased estimations of atmosphere vortices: the Saturn's storm view by Cassini VIMS-V as a case study

The size determination of dynamical structures from spectral images poses the question where to fix the shape's boundary. Here we propose a method, suitable for nearly elliptical shape, based on the fit of a 2D Gaussian to the pixel intensities of the spectral image. This method has been tested on a vortex structure embedded in the wake of the 2010 Saturn's giant storm. On January 4th 2012 the Visual and Infrared Mapping Spectrometer (VIMS), onboard Cassini, observed a giant vortex in the Saturn's northern hemisphere. The structure was embedded in the wake storm system detected on December 2010 by Fletcher et al. (2011). Therefore, all the VIMS observations focused on the Saturn's storm have been analyzed to investigate its morphology and development. VIMS detected the vortex from May 2011 up to January 2012. The shape and size evolution has been determined for the vortex cloud top, visible at 890 nm. The largest size resulted 4000 km about and seemed to shrinks continuously up to January 2012, while the shape varied in the second half of the year. The vortex oscillated in 2 degrees latitude around 37oN planetocentric latitude, and drifted in longitude by ~ 0.75 deg/day in westward direction.