Physical sciences: Circular polarization of twilight

LINEAR polarization of clear daytime sky and twilight is produced by molecular (Rayleigh) scattering and aerosol (Mie) scattering, which predominates in the infrared. Incident linearly polarized light scattered by aerosols is in general elliptically polarized, whereas no ellipticity can arise from the molecular component¹. The principal component of clear daytime sky arises from single scattering of unpolarized sunlight and is not elliptically polarized. However, whenever multiple scattering by the aerosol component is important, the strong linear polarization produced on the first scattering will lead to elliptical polarization from the second and subsequent scatterings. Although in the daytime sky this mechanism should give only a small ellipticity² (10^-5-10^-3) the possibility of a substantially larger effect at twilight arises because there is no direct illumination. We report here observations of circular polarization of twilight of order 10^-3.