Evolution and persistence of 5-mu m hot spots at the Galileo probe entry latitude

We present a study on the longitudinal locations, morphology, and evolution of the 5-mu m hot spots at 6.5 degrees N latitude (planetocentric) from an extensive Infrared Telescope Facility-National Science Foundation Camera ( IRTF-NSFCAM) data set spanning more than 3 years, which includes the date o f the Galileo probe entry. A probabilistic analysis of the data shows that within periods of several months to even more than a year, there are eight or nine longitudinal areas with high likelihood of containing a 5-mu m hot spot. These areas drift together with respect to System III at a rate which changes only slowly in time, and they are quasi evenly spaced, suggesting a wave feature. A spectral analysis of the radiance data reveals that plane tary wavenumbers 8, 9, and 10 are predominant in the data, 10 having more s pectral power in several time periods when the speed was 103.5-102.5 mis, w hile planetary wavenumber 8 has much more power when the speed is (99.5 +/- 0.5) m/s. By using the Galileo probe zonal wind speed [Atkinson et al., 19 97] at the level of the main cloud that is opaque to the radiation at 5 mu m (similar to 2 bar), our drift corrections imply a westward phase speed fo r the proposed wave. The wavenumbers and phase speeds are found to be consi stent with an equatorial Rossby wave, and the dispersive properties of this wave can account for the observed simultaneous changes in the dominant wav enumber and drift speed. We take advantage of this interpretation to infer properties of the vertical structure at 6.5 degrees N.