Abstract
Long-duration observations of Neptune's brightness at two visible wavelengths provide a disk-averaged estimate of its atmospheric aerosol. Brightness variations were previously associated with the 11-year solar cycle, through solar-modulated mechanisms linked with either ultraviolet or galactic cosmic ray (GCR) effects on atmospheric particles. Here, we use a recently extended brightness data set (1972–2014), with physically realistic modelling to show, rather than alternatives, ultraviolet and GCR are likely to be modulating Neptune's atmosphere in combination. The importance of GCR is further supported by the response of Neptune's atmosphere to an intermittent 1.5- to 1.9-year periodicity, which occurred preferentially in GCR (not ultraviolet) during the mid-1980s. This periodicity was detected both at Earth, and in GCR measured by Voyager 2, then near Neptune. A similar coincident variability in Neptune's brightness suggests nucleation onto GCR ions. Both GCR and ultraviolet mechanisms may occur more rapidly than the subsequent atmospheric particle transport.
Highlights
Long-duration observations of Neptune’s brightness at two visible wavelengths provide a disk-averaged estimate of its atmospheric aerosol
Long-term observations of Neptune from a ground-based telescope show variations in the planet’s disk-averaged brightness, which are associated with changes in the reflectivity of the planet from its atmospheric aerosol and clouds
Regular photometric observations of the magnitude of Neptune have been made since 1972, through wellcharacterised visible bandpass filters of width B20 nm centred at 472 nm (‘b’, blue) and 551 nm (‘y’, green) using a 21-in telescope at Lowell Observatory, Arizona[2], Fig. 1a
Summary
Long-duration observations of Neptune’s brightness at two visible wavelengths provide a disk-averaged estimate of its atmospheric aerosol. Brightness variations were previously associated with the 11-year solar cycle, through solar-modulated mechanisms linked with either ultraviolet or galactic cosmic ray (GCR) effects on atmospheric particles. Seasonal variations dominate the time series, Lockwood and Thompson[1] showed, using data from 1972 to 1996, that small fluctuations in Neptune’s brightness at two visible wavelengths followed the 11-year solar cycle. They examined two quantities known to vary closely with solar activity. We examine the relatively rapid fluctuations apparent during the mid-1980s in the Neptune astronomical data This enhanced variability coincided with a known episode of quasi-periodic fluctuations present in GCR13, centred around 1.68 years. Investigating Neptune’s atmospheric variability in the 1.5 to 2 year range presents a method by which to separate the two different suggested solar-modulated influences, an approach previously used to separate coincident terrestrial atmospheric responses[14]
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