A systematic bias in present models of circumsolar radiation

Abstract

Light scattering by aerosol particles is known to be fairly largest modulator of circumsolar radiation, while its contribution to the direct beam irradiance progressively increases with atmospheric turbidity. Due to their forward-lobed radiation patterns, the large aerosol particles are of special importance in understanding the shaping of diffuse light distribution in the solar aureola. The mechanism of intensive small-angle scattering is the Fraunhofer diffraction which scales up linearly with the projected area (shadow) of each single particle.Theoretically founded models of circumsolar radiation usually rely on Mie scattering – a concept exclusively applicable to spherical particles. However, the particles occurring in nature typically have nonspherical shapes with aspect ratio exceeding a few times that for a sphere (for prolate particles) or smaller than unity (for oblate particles). Here we show that models based on Mie theory systematically underestimate circumsolar irradiance compared to what we can expect from nonspherical particles. The average brightening of solar corona due to nonspherical aerosols is 10–20%, but can be as large as 40–50% for needle-like or disk-like particles. We are introducing a correction factor to improve the accuracy of the present models.