Cassini VIMS-V observations of a giant dynamical structure in the Saturn's northern hemisphere

Vortices have been observed on Saturn since the years of the Voyager's missions, included a long-lived cyclone in the southern hemisphere studied through the analysis of the Imaging Science Subsystem (ISS) cameras' images (del Río-Gaztelurrutia et al., 2010). An oval structure, about 8000 km in diameter size and ~ 0.83 eccentricity in the Saturn's north hemisphere, has been observed by the Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini spacecraft on January 4th 2012. The vortex is centered at an average planetocentric latitude of 35.5 °North, inside the storm system detected at the end of 2010 (Fletcher et al, 2011). To find the first occurrence of this vortex we started the examination of both VIMS and ISS databases from the 2010 fall until the end of January 2012. The archive of ISS narrow angle (NAC) and wide angle (WAC) cameras, publicly available from the NASA Planetary Data System (PDS) Imaging Node, gave us images both VIMS concomitant and time shifted to check the persistence of the dynamical structure. We adopted the same identification criterion used by del Río-Gaztelurrutia et al. (2010), by searching for an oval of analogous dimension in the same zonal region. This vortex has been observed at different distances and viewing geometries at least 5 and 4 times by VIMS and ISS, respectively, in the examined time period. In this work we aim to determine the oval identity in a univocal way, on the basis of its position and size, in order to monitor both the structure living cycle and its evolution. We describe the position and size of the vortex for each image with the greatest detail by means of different processing and mapping techniques. Our results show that there were some zonal drift and size and shape evolution in the time period of our survey. In a second poster presentation (Oliva et al., 2012, P13B-1938) the altitude and the optical depth variation of the cloud at the top of the dynamical structure, visible in the 0.89 ?m CH4 absorption band, is analyzed.