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G. W. Lockwood (Lowell Observatory)
Photometry of Neptune from 1950-1966 in B (440 nm) and 1972-2001 in b (472 nm) and y (551 nm) shows that as the planet approaches its 2007 southern summer solstice its disk-averaged albedo is presently greater than at any time during the past half century. Neptune has brightened by 11% in b and 10% in y since 1980 with most of the increase occurring in nearly equal yearly increments beginning in 1990. A smaller than average increase from 2000 to 2001 may signal an imminent end to the brightening trend.
The nature of the year-to-year variations changed after 1990 when a steady rise overshadowed an inverse correlation with cyclic solar activity. That correlation remains statistically significant over the 30-year time series, 1972-2001, for the varying solar Lyman alpha flux but not for the galactic cosmic ray flux modulated by solar activity. Therefore a photochemical mechanism is now favored over a particle-induced condensation process.
Comparison of visible and near infrared photometry during the 1976 infrared outburst reveals similar patterns of variation, but with 20-50x smaller amplitude at visible wavelengths. Other comparisons in 1989 with ground-based and Voyager images reveal a photometric signature associated with disk transits of the Great Dark Spot/Bright Companion complex. The long-term record shows that (1) the 1976 outburst was a unique event; (2) Voyager's arrival at Neptune occurred at the end of a three-year interval of enhanced atmospheric activity during which discrete features perhaps even larger than the GDS/BC complex were continually present.
This work is supported by NASA Planetary Astronomy grant NAG5-7853.