Solar cycle relationship clouded by Neptune's sustained brightness maximum

Abstract

FOR almost two decades, Neptune's brightness varied inversely, at the level of a few per cent, with the solar cycle. The anticorrela-tion was so striking that some causal mechanism seemed necessary, and several suggestions were made^1,2. Two different but plausible ideas involving solar-induced global changes in Neptune's atmosphere were a cyclic darkening ('tanning') of stratospheric aerosols caused by varying ultraviolet radiation³ and a variation in the rate of ion-induced nucleation of atmospheric aerosols due to the modulation of galactic cosmic-ray flux by solar activity⁴. In 1990, with the current solar cycle near its peak, however, Neptune departed unexpectedly from the previous cyclic behaviour, attaining its greatest brightness since 1972. Further observations will be needed to decide if the present deviation signals a unique atmospheric phenomenon, and to see if the cyclic anticorrelation will be restored.