Abstract

Near-forward scattering by cloud droplets is known to give rise to colored rings, centered on the Sun or the Moon, which are called the corona. Because of the spherical shape of the droplets, the corona can be circular. A Finnish amateur astronomers' network has found a corona, in a seemingly cloudless sky, with a regular vertically elliptical shape. The aspect ratio of these ellipses changes with the altitude of the Sun or the Moon. Some brightening in the coronas has also been reported. Because of observations of high pollen concentrations at the time of occurrence of these coronas, we propose that some coronas can be explained as a result of scattering by birch pollen grains, which are more or less spheroidal. To explain other observed coronas, pollen grains with more complicated shapes, such as pine and spruce pollen grains, must be invoked. Our analysis is limited to spheroidal grains, for which the Fraunhofer theory gives analytical expressions of simple form. The more complicated shapes require numerical simulations or laboratory experiments, which we have not done.

© 1994 Optical Society of America

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