Observations and Analysis of Longitudinal Thermal Waves on Jupiter

We analyze the properties of wave-like longitudinal temperature variations in Jupiter's upper troposphere and stratosphere, as revealed using thermal infrared (IR) ground-based observations taken yearly during the 1989–1993 period. These thermal waves are apparently the same as the “slowly moving thermal features” identified in Voyager data. We conclude that they are ubiquitous at near-equatorial latitudes on Jupiter, and have a spatial scale which typically spans wavenumber ≊2 to ≊15. Their stratospheric (20 mbar) temperature amplitude, from 7.8 μm data, is approximately a factor of 3 larger than their amplitude in the upper troposphere (≊0.3 bars), as revealed using 18 μm data. These amplitudes are consistent with the ρ −1/2growth expected for a vertically propagating Rossby wave. The phase at the two levels is the same to within 5° of longitude. The temperature wave is not advected by the zonal winds, having a phase velocity of ≊5 m sec −1in system III, but the temperature amplitude can decay on time scales ≊10 5sec. Imaging of the thermal waves shows that they are extended in latitude to a greater extent than the belt/zone structure, or the zonal wind jets. An apparently related wave is seen in broad-bandwidth (7–13 μm) IR observations which are sensitive to clouds and aerosols. This structure is advected by the zonal wind, indicating that some cloud or aerosol component shares the wave-like structure and spatial scale of the temperature wave. We interpret these waves as stationary Rossby waves, represented by small latitude excursions in the streamlines of the zonal winds. The latitude excursions could be produced by either deep structure, or by the global effect of vortices such as the Great Red Spot. In either case, temperature fluctuations are produced via “vortex stretching.” We calculate the required magnitude of the latitude excursions in the zonal winds to be ≊1° in order to produce the observed temperature amplitudes. This is a subarcsec scale for Earth-based observations, but might be detectable using observations from the Hubble space telescope.