Polarization of light from noctilucent clouds

Abstract

The polarization of light from noctilucent clouds has been measured in two spectral regions by a photographic technique. The photographs were made at Torsta (63.3°N, 14.6°E) in central Sweden. The measured cloud features were selected from a cloud-covered area of the order of 105 km2, and the exposures were made over a period of 3 hours. No significant variations were observed in time or space in the behavior of the polarization. The particles forming the clouds may be of irregular shape. Since their orientation is likely to be random, it is reasonable to assume that they can be represented by spherical particles of comparable size. In this paper, following previous work, the light is assumed to be singly scattered sunlight, and the Mie theory of scattering by spherical particles is applied in order to derive the particle radius from the observed polarization. For all observed scattering angles (20° to 60°) the degree of polarization was slightly less than that given by Rayleigh's law, and the values in the red wavelength region were found to exceed those in the blue. If the clouds are regarded as a monodisperse aerosol, comparison of the data with theoretically computed curves indicates that the radius of the particles ranges from 1.0×10−5 cm to 1.8×10−5 cm, depending on the refractive index assumed. For the more plausible assumption of a polydisperse particle distribution the results indicate that no appreciable proportion of particles with radii larger than 2.4×10−5 cm can be present in the clouds.