On the phenomenon of the blue Sun

Abstract. This study examines the cause of the blue colour of the Sun as observed after the eruption of Krakatoa in 1883 as well as other volcanic eruptions or massive forest fire events. Aerosol particles, e.g., volcanic ash or products of biomass burning are believed to be able to modify the spectral distribution of transmitted solar radiation making it appear blue or green to a human observer. Previous studies already showed that narrow aerosol particle size distributions with radii on the order of about 500 nm can lead to anomalous scattering, i.e., scattering cross sections increasing with increasing wavelength in the visible spectral range. In this work we treat the effect of Rayleigh scattering on the shape of the transmitted solar spectrum correctly employing radiative transfer (RT) simulations with the SCIATRAN RT-model. The colour associated with solar transmission spectra is determined based on the CIE colour matching functions and CIE chromaticity values. It is shown that a blue Sun can be simulated for aerosol optical depths (at 550 nm) of about τ = 0.5 (or higher) if Rayleigh scattering is taken into account. Without considering Rayleigh scattering – as in most of the previous studies – a blue Sun is in principle produced with aerosol optical depths as low as about τ = 0.1 (at 550 nm), if the aerosol particle size distribution is chosen to maximize anomalous scattering in the visible spectral range. It is demonstrated that Rayleigh scattering – as expected – has a strong impact on the transmission spectrum, particularly at low solar elevation angles, and needs to be considered for a correct determination of the perceived colour of the Sun. We also test the hypothesis that the blue Sun after the eruption of Krakatoa was caused by large abundances of water vapour in the atmosphere, as proposed in earlier studies. In addition, we present a case study on a particularly noteworthy blue-Sun-event in the past, i.e., the one related to the large Canadian forest fires in September 1950.