Fathoming solar effects in Neptune’s atmosphere

Long-duration observations of Neptune’s brightness in 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 ultra-violet (UV) or galactic cosmic ray (GCR) effects on atmospheric particles. Here we use a recently extended brightness dataset (1972-2014), with physically realistic modelling to show that rather than alternatives, UV 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 UV) 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 UV mechanisms may occur more rapidly than the subsequent atmospheric particle transport. Introduction Long-term observations of Neptune from a ground-based telescope show small variations in the planet’s disk-averaged brightness, which are associated with changes in the reflectivity (albedo) of the planet from its atmospheric aerosol and clouds. Although seasonal variations dominate the time series, Lockwood and Thompson1 showed, using data from 1972-1996, 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. The first, solar ultraviolet (UV) radiation , is linked to photochemical variations in Neptune’s atmospheric aerosol particles, and the second, galactic cosmic rays (GCR) , may create some of Neptune’s aerosol through ion-induced nucleation. It was not possible to discriminate between the UV and GCR effects, though the relationship with UV was slightly more statistically robust. The Neptune magnitude-solar activity relationship broke down after 1996[1,2], but recent extension of the data3 encourages its re-examination. Supporting evidence for a solar cycle in infra-red observations from 1975-2007[4] further motivates a fresh consideration of the origin of the short-term variability in Neptune’s albedo. Both the UV and GCR mechanisms can, in principle, account for the changes observed in the photometric observations, which originate in Neptune’s stratosphere and troposphere,5. The UV mechanism was originally proposed6 to explain the solar cycle signal when it was first reported in Neptune’s albedo7. It was suggested that UVtriggered surface chemistry on pre-existing aerosol particles varied the optical properties of Neptune’s stratospheric aerosol through a darkening in colour (“tanning”), detectable in the photometric measurements. The GCR-driven mechanism was proposed for Neptune8,9, through direct (“Wilson”) condensation of supersaturated gas onto atmospheric ions10, causing particle growth ultimately detectable at optical wavelengths. The possibility that charge-related effects could modulate the atmospheres of the outer planets, where variations in solar irradiance are proportionately less important11, contrasts with the likely role of energetic particles in Earth’s atmosphere12, and provides a further motivation for this study. The two proposed mechanisms for external solar forcing of Neptune are essentially heliospheric (through GCR) or photospheric (through solar UV) in origin. The analysis to investigate them here uses two approaches. Firstly, the relationships between Neptune’s magnitude, solar UV radiation and GCRs are studied with multiple regression, allowing both the proposed mechanisms to act together. We find that, when the extra degrees of freedom are accounted for, including both UV and GCR improves prediction of the magnitude fluctuations. Secondly, 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 variability present in GCR13, centred around 1.68 years. Investigating Neptune’s atmospheric variability in the 1.5 to 2 year range therefore presents a method by which to separate the two different suggested solar-modulated influences, an approach previously employed to separate coincident terrestrial atmospheric responses14.