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L. A. Sromovsky, P. M. Fry (University of Wisconsin - Madison)
HST observations of Neptune at 467 nm reveal a brightness increase between 1996 and 2002 that occurred mostly at southern mid latitudes (Sromovsky et al., Icarus \bf 163, 256-261, 2003) and produced a disk-averaged increase of 3.2%. This extended a 10% increase observed since 1980 at 472 nm (Lockwood and Thompson, Icarus \bf 156, 37-51, 2002). To interpret this variation we developed a simplified analytical seasonal model for Neptune that combines a phase-shifted local albedo response to solar forcing with the varying viewing and illumination geometry (Sromovsky et al. 2003). We have since explored a number of numerical models, with restricted bands of activity and a variety of response functions. A model with bands at 20-70 S and 20-70 N, antisymmetric albedo variations, and a 30-year response delay is most consistent with current observations. This model produces both bright and dark bands in both hemispheres, with simulated images that are approximately consistent with HST and Voyager observations. Although the blue albedo of Neptune increased by 10% between 1980 and 2000, the near-IR albedo actually decreased, a puzzling result that may be partly explained by the fact that near-IR observations are most sensitive to high altitude cloud features that seem to dominate the band brightening, but do not respond to the dark phase of the bands, which only affect I/F at short wavelengths. New groundbased near IR imaging observations at the IRTF and Keck telescopes in July and August 2003 will track expected further increases in bright cloud activity. Confirmation of an expected 467-nm darkening at northern latitudes must await an opportunity for new HST observations.
This research was supported by NASA Grant NAG5-12012 and HST Archive Research Grant AR-0922.01-A. Based on observations made with the NASA/ESA Hubble Space Telescope. STScI is operated by AURA, Inc., under NASA Contract NAS 5-26555.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.