Velocity and vorticity measurements of Jupiter's Great Red Spot using automated cloud feature tracking

Abstract

We have produced mosaics of the Great Red Spot (GRS) using images taken by the Galileo spacecraft in May 2000, and have measured the winds of the GRS using an automated algorithm that does not require manual cloud tracking. Our technique yields a high-density, regular grid of wind velocity vectors that is advantageous over a limited number of scattered wind vectors that result from manual cloud tracking. The high-velocity collar of the GRS is clearly seen from our velocity vector map, and highest wind velocities are measured to be around 170 m/s. The high resolution of the mosaics have also enabled us to map turbulent eddies inside the chaotic central region of the GRS, similar to those mapped by Sada et al. (1996) and Vasavada et al. (1998). Using the wind velocity measurements, we computed particle trajectories around the GRS as well as maps of relative and absolute vorticities. We have discovered a narrow ring of cyclonic vorticity that surrounds the main anti-cyclonic high-velocity collar. This narrow ring appears to correspond to a ring surrounding the GRS that is bright in 5-microns (Terrile and Beebe 1979). It appears that this cyclonic ring is not a transient feature of the GRS, as we have discovered it in a re-analysis of Galileo data taken in 1996 first analyzed by Vasavada et al. (1998). We also calculate how absolute vorticity changes as a function of latitude along a trajectory around the GRS and compare these measurements to similar ones performed by Dowling and Ingersoll (1988) using Voyager data. We show no dramatic evolution in the structure of the GRS since the Voyager era except for additional evidence for a counterrotating GRS core, an increase in velocity in the main velocity collar, and an overall decrease in the length of the GRS.