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E. Garcia-Melendo (Esteve Duran Observatory Foundation, Spain), A. Sanchez-Lavega (Universidad del Pais Vasco, Spain), J. M. Gomez (GEA, Spain), J. Lecacheux (Obs. Paris-Meudon, France), F. Colas (Bureau des Longitudes, France), I. Miyazaki (Okinawa, Japan), D. Parker (Florida, USA)
A long-lived series of spots have been observed regularly in the North Temperate Belt (NTB) of Jupiter following the 1990 large-scale disturbance (Sanchez-Lavega et al., 1991). These features have been tracked using more than 20000 CCD images taken with several ground based telescopes complemented with those from the HST imagery archive, spanning a period from 1991 up to the end of 1998 (spectral range ~ 300 to 890nm). Our data indicate that 7 of these features had a lifetime of 4 years although some of them could have survived since their formation in 1991. These features are located within a few tenths of degree southwards of the 23.7N high-speed jet peak and represent a record of longevity for any feature observed at the latitude of the NTB jet. The high resolution WFPC2 HST images show that these spots are week anticyclones, with an average relative vorticity of 2.0(±0.8)x10-5s-1, a value close to the ambient vorticity of the NTB jet at this latitude. The vortices move with an average zonal velocity of 125ms-1. The zonal wind profile of the 23.7N jet between 1995 and 1997 was also measured using both, cloud tracking and a correlation technique based on reflectivity zonal profiles. It was found that the amplitude as well as the shape of the jet was clearly different from the observed one by Voyager 2 in 1979. The jet peak speed is about 30ms-1 slower, is broader in latitude, and shows signs of unstability against the barotropic mechanism. This could explain the nature of the vortices. Sanchez-Lavega, A., Miyazaki, I., Parker, D., Laques, P., Lecacheux, J., 1991, A disturbance in Jupiter's High-Speed North Temperate Jet during 1990, Icarus 94, 92-97