The Structure and Temporal Stability of Jupiter's Zonal Winds: A Study of the North Tropical Region

This paper investigates possible long-term zonal wind variations using Voyager (1979), Hubble Space Telescope (HST; 1995, 1997, 1998), and Galileo Orbiter (1997) data that has all been processed and analyzed in the same manner for consistency. Previous attempts to measure the zonal wind profile in Voyager data had yielded inconsistent results for the maximum eastward jet near 23.7° N latitude from different studies (A. P. Ingersoll et al., 1979, Nature280, 773-775; ibid 1981, J. Geophys. Res.86, 8733-8743; T. Maxworthy 1984, Planet. Space Sci.32, 1053-1058; S. Limaye 1986, Icarus65, 335-352). This paper compares previous measurements and includes new independent analysis to conclude that the correct wind jet velocity at the time of Voyager is close to 182 m s^-1, most consistent with the reported results of Maxworthy (1984). In the data sets from different epochs the overall shape of the wind profile and locations of the prograde and retrograde jets remain constant, but there are amplitude changes at a few latitudes. The largest velocity deviations between the Voyager and the 1995-1997 HST profiles occurred in the maximum eastward jet, near 23.7° N, and at 6° S and 20° S planetographic latitude. Possible causes for an apparent zonal wind decrease were explored using Galileo visible and HST near-infrared data, and the wind velocity change was determined to be real. A very definite change in cloud structure and brightness has also occurred at 23.7° N. Additionally, surface brightness changes seen at 6° S and 20° S also correlated with changes in wind speed. HST WFPC2 data obtained in 1998 showed the recovery of the 23.7° N region to a Voyager-like appearance and an increase in wind velocity to the Voyager value.