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G. W. Lockwood (Lowell Observatory), H. B. Hammel (Space Science Institute)
The light curve of Neptune, measured on more than 300 nights since 1972 at the Lowell Observatory using b (472 nm) and y (551 nm) interference filters, displays two distinct patterns. Before 1989 the brightness rose and fell, seemingly in response to solar activity or some other low frequency stimulus (G. W.Lockwood and D. T. Thompson 1991, Nature 349, 593). Beginning around 1989 the brightness has increased steadily by 0.6-0.8% per year and the cumulative increase since 1971 is now more than 10%. Only a minuscule fraction can be accounted for by Neptune's changing aspect as seen from Earth; most of the variation is due to varying amounts of clouds and global haze.
During the Voyager enounter in 1989, Neptune's appearance was dominated by the Great Dark Spot and its associated Bright Companion. Ground-based observations during the encounter allow comparison of these features with their photometric signatures. At visible wavelengths, the photometric effect of the GDS transit is quite small (no more than 1-2%), but changes in the methane bands are much larger and have been measured from Voyager images at 619 nm, Lowell spectrophotometry near 619 and 726 nm, Perth Observatory photometry at 726 nm, and Mauna Kea CCD images at 889 nm.
Recent Hubble Space Telescope imaging from 1994 to 1998 and near-IR images obtained in August 2000 using the Keck Observatory adaptive optics system allow comparisons of photometry and images over much of the epoch of steadily rising planetary brightness. We expect this comparison to provide a more comprehensive view of Neptune's long term atmospheric changes.
This work was supported by grants from NASA and STScI.