The main apparent wave features observable in Neptune's atmosphere are the South Polar Wave (SPW), and possibly transient cloud features known as "outbursts". Other examples include the observed oscillation of discrete cloud features that was discussed in Section 5.8.2.
A dark, apparently axisymmetric, feature was detected near Neptune's South Pole by Voyager 2 in 1989 (Figure 5.43, see color section). The band extended between planetocentric latitudes of 65°S and 40°S. However, within this band was a narrower, darker region that appeared as a wavenumber-1 planetary wave. The poleward excursion of the inner band was bordered on the north by a dark anticyclone (DS2) centered at 55°S, while the longitude of maximum equatorial excursion was marked by an outbreak of bright clouds at 70°S, the SPF. This wave appears in many respects to be analogous to the North Polar Hexagon in Saturn's atmosphere, which is believed by some to be a planetary wave forced by interaction with its own equator-side anticyclone, the NPS (although the NPS has now disappeared). However, while the North Polar Hexagon is quasi-stationary with respect to System III, suggesting a link with the rotation of the bulk interior, Neptune's SPW was estimated to drift significantly with respect to interior rotation (Sromovsky et al., 1993). Since the Voyager encounter, HST and ground-based telescopes between 1994 and 1998 (Sromovsky et al., 2001c) showed that while the SPW was still clearly visible, the latitudinal extent of the band had shrunk to between 55°S and 65°S. In addition, the SPF was rarely apparent and DS2 had completely disappeared. Dynamically, Sromovsky et al. (2001c) suggest that it is hard to see how such a wave could still be present in the absence of a forcing anticyclone and suggest that DS2 may have in fact still been present, but was too faint to observe. However, it may also be possible that the SPW is simply caused by barotropic instability and requires no forcing anticyclone. A dark band continues to exist at this latitude and still had a Wavenum-ber-1 characteristic at least until 2002 (Larry Sromovsky, pers. commun.).
As mentioned in Section 5.8.2, while the Voyager GDS had disappeared by 19941996, a new dark feature NGDS-32 had appeared at 32°N, which remained fixed in latitude during the observation campaign. In 1994 many bright clouds were observed extending from 30°N to the equator at roughly the same longitude as NGDS-32 (Sromovsky et al., 2001c). Furthermore, the zonal velocity of these clouds was substantially less than the average zonal velocity at the equator and instead the speed seemed locked to the zonal velocity at the NGDS-32 latitude. This sudden appearance of bright equatorial clouds (Sromovsky et al., 2001b) would seem to be similar to the "outbursts" postulated to explain the increased disk-averaged albedo observed with ground-based telescopes between 1986 and 1987 by Hammel et al. (1992) and likened to the GWS of Saturn. A similar link between near-equatorial dark spots and equatorial clouds was observed by Voyager 2 where a bright "smudge" was observed just past the equator at the same longitude as the GDS and apparently co-rotating with it. This co-rotation suggests some sort of wave interaction and since the phase speed of the wave would appear to be eastward the most likely candidate is an equatorially trapped Kelvin wave (Sromovsky et al., 2001c). However, the equatorial confinement of such a wave with an estimated phase speed of 210 m s_1 is calculated from the equatorial deformation radius to be 17° of latitude while the observed clouds actually extended to 30°N. Hence, the nature of this interaction clearly warrants further study.
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