The Ring effect in skylight polarisation

Skylight reflectivity and polarisation spectra across the ultraviolet (UV) and visible (VIS) wavelength range show abundant features coinciding with solar Fraunhofer lines. Although the appearance of these features in reflectivity spectra has been shown to be caused by rotational Raman scattering by atmospheric molecules, quantitative modelling of this so-called Ring effect as observed in polarisation spectra of scattered sunlight is still lacking. Accurate modelling of UV-VIS radiances and polarisation at high-spectral resolution has, however, become increasingly important because of the increased use of high-spectral resolution spectrophotometers, that are often highly polarisation sensitive. In this paper, we present high-spectral resolution skylight polarisation spectra and show that the Ring effect in these spectra can be simulated using a radiative transfer program that includes rotational Raman scattering up to the second order of scattering.