Polarimetric studies of rotational Raman scattering (the Ring effect) in the zenith sky

Polarimetric spectra of the light from the zenith daytime sky have been recorded using a two dimensional detector (CCD) with the aim of measuring the degree of linear polarization, p, across Fraunhofer lines present in the original spectrum of the incoming solar radiation. They reveal that p(λ) has structure through a line with a form which changes during the course of a day. Generally p is reduced in the core of the lines but enhanced on their shoulders. For the few weeks around midsummer at the Glasgow site, there is a short time near local noon when the p(λ) structure virtually disappears with p even exhibiting an enhancement in the line-cores at the time of the solar meridianal transit. this behaviour is unaccountable by Rayleigh scattering for which p is wavelength independent but the recorded phenomena can be explained by rotational Raman scattering by the molecules (N2 and O2) in the Earth’s atmosphere, this also being the chief mechanism for the photometric line filling-in or Ring effect. The polarization dip features result from Raman scattered flux being shifted in wavelength from the continuum to within the line core. In respect of the line-core polarization enhancements recorded at small solar zenith angles during the summer, this again can be explained by the Raman mechanism but with secondary Rayleigh/Raman and Raman/Rayleigh scattering being required. ck of consensus of previous reports of the temporal behaviour of the Ring effect as measured by simple spectrophotometry is also discussed. It is suggested that the disparities between the various observers may well result from the sensitivity of the different photometric instruments to polarized radiation and hence to the line polarization behaviour as reported in this paper.