Prior to the arrival of the Cassini spacecraft in 2004, a number of waves had been seen in Saturn's atmosphere. Achterberg and Flasar (1996) analyzed Voyager IRIS mid-IR observations to find wavenumber-2 waves near the tropopause at northern midlatitudes between 20°N and 40°N, which were quasi-stationary with respect to Saturn's interior (as defined by System III). However, these waves were dwarfed by two major planetary-scale waves observed at visible wavelengths by Voyager ISS: the

Figure 5.30. The "Ribbon Wave'' cloud structure in Saturn's atmosphere observed by Voyager 2 with a "green" filter. Courtesy of NASA.

"Ribbon Wave'' at 41 °N (planetocentric, 46° planetographic), and the North Polar Hexagon associated with the jet at 74°N (planetocentric, 77° planetographic).

The Ribbon Wave was first detected by the Voyager 2 spacecraft in 1981 (Smith et al., 1982) and appeared at visible wavelengths as a thin wavy line within the bright zone at 41°N (planetocentric, 46°N planetographic), coincident with the first major eastward jet in Saturn's atmosphere north of the equatorial jet (Figures 5.30 and 5.31, see color section for latter). The wavelength of the Ribbon Wave was of the order of 5,000 km and the phase speed was approximately 140 ms-1. The wave feature was Fourier-analyzed by Sromovsky et al. (1983) and later Godfrey and Moore (1986), who found that the dominant wavenumbers were 8 and 20. Although both sides of the zone were equally bright in green and red filters, the southern side appeared brighter in violet images. However, this contrast was completely reversed in the UV, where the southern side appeared almost black. These observations pointed to the existence of a high-altitude haze of small particles to the south of the wave and low haze opacity to the north. This suggested that air was rising to the south of the Ribbon Wave and sinking to the north, consistent with cloud tracking of features that showed the northern edge to be cyclonic and the southern edge to be anticyclonic. Sromovsky et al. (1983) derived a dispersion relation for the Ribbon Wave, which they argued showed that the wave was fundamentally a barotropic Rossby wave. However, Godfrey and Moore (1986) pointed out that the zonal wave curvature d2u/dy2 (for which high values are expected to lead to barotropic instability) was at a minimum at the central latitude of the wave and this appeared to be inconsistent with the barotropic wave hypothesis. Instead, they favored a baroclinic Rossby wave explanation since the Voyager IRIS measurements showed the temperature at 150mbar (Hanel et al., 1982) to have a sharp gradient at this latitude with temperatures to the south being roughly 6 K cooler than the north. This is again consistent with the picture of rising air to the south and descending air to the north. The position of the wave at UV wavelengths (where we only see high in the atmosphere) was identical to that in visible wavelengths, indicating little vertical shear in the feature.

The Ribbon Wave was observed again in 1995 by HST (Sanchez-Lavega, 2002) with the same wavelength and phase speed and thus appeared at the time to be a long-lived feature. However, Cassini has found no trace of the Ribbon Wave, although numerous other small wave features, due to Kelvin-Helmholtz instabilities, have been detected on many belt/zone boundaries (Figure 5.32). These are regions of strong thermal gradient, and thus strong wind shear, where instabilities would appear to lead to the formation of waves. Hence, the Ribbon Wave does not now appear to be a special feature, but rather a particularly strong and prominent example of a class of waves that occur all the time in Saturn's atmosphere, which happened to be particularly visible during the Voyager observations and in the early 1990s. In fact, ground-based monitoring reveals that at the start of the Cassini observations in 2004, Saturn was going through a period of peculiarly negligible zonal wave activity, although this seemed to be increasing again in 2007. In addition, prior to Cassini no wave features had been observed in the southern hemisphere, but wave features have now been observed in both hemispheres.

Apart from waves seen at visible wavelengths on belt/zone boundaries, Cassini has made observations of other types of waves. One of these is the "String of Pearls'' seen by Cassini VIMS at ^40°N, which is due to variations in optical depth of the main (presumably ammonia) cloud deck and thus reveals motions in the 2 bar to 3 bar region (Figure 5.33, see color section). More than 24 bright spots appeared at this latitude in April 2006 separated by about 3.5° longitude. This is the first time that such a long and regular train of cloud clearings has been seen and would appear to be a manifestation of a large planetary wave. One of the goals of the Cassini mission was to probe the thermal structure of planetary waves to get a better understanding of their dynamics. Since wave activity has been relatively light during Cassinis mission so far there has been limited opportunity to do this. However, the Cassini CIRS instrument has seen a wave-like feature at 45°N to 50°N, which appears as a series of

Figure 5.32. Kelvin-Helmholtz instability waves seen at a belt/zone boundary of Saturn by Cassini ISS in 2004. Courtesy of NASA.

hotspots, spaced by 30°, originating from the 100 mbar to 500 mbar level. This feature is still being investigated. Orton and Yanamandra-Fisher (2005) also saw zonal temperature oscillations near 32°S (planetocentric), consistent with a wave of wavenumber-9 or wavenumber-10.

Although the Voyager flybys of Saturn occurred in 1980 and 1981, the second major wave system after the Ribbon Wave, the "North Polar Hexagon'' was not discovered until 1988, when Voyager 2 images were re-projected to produce polar maps (Godfrey, 1988). The feature revealed by this re-projection, shown in Figure 5.34, was a very regular wavenumber-6 wave centered in a 100 ms"1 eastward jet at 76°N (planetographic). On the southern edge of one of the Hexagon's faces was the North Polar Spot (NPS). What is most remarkable about these features is that they were found to remain almost static with respect to the System III rotation frame, which has a period close to Saturn's bulk rotation rate (see Section 2.7.3). It seems unlikely that the magnetic field itself could be exerting such an influence on the motion of the observable troposphere and it thus appeared that both the NPS and the Hexagon were linked in some way to the deep interior. Allison et al. (1990) suggested that the Hexagon was a stationary Rossby wave forced by interaction between the eastward jet and the adjacent NPS and meridionally trapped by the strong relative vorticity gradient of the flow itself. Both the NPS and NPH were observed again in 1990 with ground-based observations (Sanchez-Lavega et al., 1993) and HST (Caldwell et al., 1993). Sanchez-Lavega et al. (1993) found that the appearance of the NPS had changed in the intervening 10 years. Voyager observed that the NPS had greatest contrast at 419 nm, but was barely visible at 566 nm. However, in 1990 the NPS was observable from yellow to red wavelengths and

Figure 5.34. Saturn's North Polar Hexagon, North Polar Spot, and Ribbon Wave as observed by Voyager 2. Polar stereographic projection after Godfrey and Moore (1986). The Ribbon Wave at 47°N is clearly visible. The Hexagon Wave is not so clear, but is visible at latitude —78°N. The North Polar Spot is at latitude 75°N and longitude (planetographic) of 320°W.

Figure 5.34. Saturn's North Polar Hexagon, North Polar Spot, and Ribbon Wave as observed by Voyager 2. Polar stereographic projection after Godfrey and Moore (1986). The Ribbon Wave at 47°N is clearly visible. The Hexagon Wave is not so clear, but is visible at latitude —78°N. The North Polar Spot is at latitude 75°N and longitude (planetographic) of 320°W.

was particularly bright in the near-IR methane bands, indicating a high cloud top of —90mbar. An additional finding was that while the Voyager and 1990 observations estimated the NPS to be drifting with longitude at similar rates, extrapolating the position of the Voyager NPS forward to 1990 predicted a position approximately 60° away from where the NPS was actually observed. Sanchez-Lavega et al. suggested that an explanation for the observed differences might be that there were in fact two spots, on two adjacent sides of the Hexagon, which alternately brightened and faded. Since the NPS was seen to drift so slowly with respect to System III, it was suggested that the NPS might in some way be fixed to the deep interior and its presence deflected the jet stream around it, setting up the NPH wave system. However, the internal rotation rate of Saturn is the source of some speculation, as we saw earlier in Section 2.7.3, since we now know that the System III rotation rate varies with time and is not rigidly linked to interior bulk rotation (Anderson and Schubert, 2007; Gurnett et al., 2005). Hence, the relationship between the NPH and the interior is not clear. An alternative and probably more plausible explanation of the NPH (Aguiar et al., 2008) is that the zonal winds at the NPH latitude of 77°N violate the Rayleigh-Kuo criterion for barotropic instability (i.e., the latitudinal gradient of potential vorticity changes sign at the latitude of the jet; Section 5.3.1) and thus the NPH results from a finite-amplitude, nonlinear equilibration of a barotropic instability of the jet.

The Cassini mission has made extensive observations of both poles of Saturn and the warm cyclonic vortices found there were discussed in Section 5.6.1. Since Saturn's North Pole has been in darkness since Cassini arrived it has not been possible to establish whether the NPH and NPS are still detectable at visible wavelengths. However, it has been possible to observe the 5 ^m thermal emission from the deep atmosphere (which maps the variation in optical thickness of the main cloud deck at 2-3 bar) with the VIMS instrument and also the thermal emission from the upper troposphere up to the stratosphere with Cassini CIRS. Remarkably the NPH has been observed by both Cassini instruments. Fletcher et al. (2008a) found a warm cyclonic hexagon-shaped belt in the upper troposphere (800-100 mbar) at 79°N (planetographic) in CIRS observations (Figure 5.25), which coincides with a 5 ^m bright hexagon seen by VIMS (Figure 5.35). Surrounding this feature, a cold anti-cyclonic hexagon-shaped zone is seen by CIRS at 76°N (planetographic), which coincides with a 5 ^m dark hexagon seen by VIMS. Fletcher et al. (2008a) suggest that upwelling on the equatorial side of the polar jet and subsidence on the poleward side might be responsible for both the warming at 100 mbar to 800 mbar and also the cloud clearing at 2 bar to 3 bar leading to the 5 ^m bright hexagon seen in VIMS images. The NPH thus appears to be an extraordinarily long-lived feature that appears to extend from the deep atmosphere right up to the tropopause. The fact that this feature exists over such a great altitude range supports the theory that it is due to barotropic instability, but why the wave should be wavenumber-6 and not any other is unclear, although it is interesting to note that the wavelength is very similar to the mean distance between Saturn's belts and zones and may be a "preferred length'' of the atmosphere. However, although the NPH has been rediscovered by Cassini, neither CIRS nor VIMS detected the NPS, although VIMS did observe numerous vortices outside the NPH, one of which might perhaps be associated with the NPS. Further mapping of the area at visible and near-IR wavelengths will be an important objective once sunlight returns to the North Pole in 2009.

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