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L. A. Sromovsky, P. M. Fry (U. of Wisconsin), T. E. Dowling (CPL/U. of Louisville), K. H. Baines (JPL)
Voyager 2 imaging of Neptune in 1989 revealed an earth-sized southern Great Dark Spot (GDS) with an associated bright poleward companion cloud. Since 1994 two new dark spots have been observed in the northern hemisphere. One near 32\circ N was seen first in 1994 (Hammel et al. 1995, Icarus \bf 265, 1740) and last in 1996 (Sromovsky et al. 2000a Icarus, submitted), always with a bright poleward companion. The second was seen in 1996, with no companion, at a surprisingly low latitude of 15\circ N (Sromovsky et al. 2000a).
While the 1989 GDS drifted equatorward, as expected from simulations of anticyclones in a background anticyclonic shear (e.g. Lebeau and Dowling 1998, Icarus \bf 132, 239), neither of the northern spots has exhibited latitudinal drift. A possible explanation is that there are local variations in zonal wind or temperature structure that modifies the potential vorticity gradient in a way that provides stability against drift. New circulation measurements based on 1998 HST observations (Sromovsky et al. 2000b Icarus, submitted), combined with earlier wind measurements from 1995 and 1996, do show regions of negative absolute vorticity gradients, but not at latitudes that would explain the lack of dark spot drift.
The drift of the 1989 GDS toward the equator likely led to its dissipation before the fall of 1990. The new dark spots seem to have found some other mechanism for decay. Between 1996 and 1998 the two new dark spots both disappeared from view (Sromovsky et al. 2000b). However, because the companion clouds are still prominent in 1998, we believe that NGDS-32 still exists as an anticyclone, even though not visible as a dark feature in the F467M (blue-filter) images. The bright cloud seen at ~40\circ N in July 2000 IRTF images of Neptune suggests that NGDS-32 might still exist.
This work was supported by STScI grants GO-07324.01 (LAS), GO-07324.02 (TED), and GO-7324.03(KHB), and NASA Grant NAG5-6788 (LAS), and is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the STScI, which is operated by AURA under NASA contract NAS5-26555.