Abstract

The corona is a diffraction phenomenon that, in its most spectacular form, is manifested by a concentric series of three or four brilliantly colored rings about the sun or moon. In its simplest form, the corona (literally a crown) is represented by the aureole (or halo of the magico-religious variety), which is a white disk boarded near the sun or moon by a bluish ring and terminated by a reddish brown band. Iridescence is a related display whose patches of brilliant color, usually seen at large angular separations from the sun, can be considered as fragments of corona rings. To obtain a unified understanding of these phenomena, it is only necessary to consider basic diffraction theory and have general knowledge of the microphysical contents of the clouds causing the displays (see, e.g., Sassen 1979). According to simple diffraction theory, the purest effects are produced by monodispersed particle sizes, with the radius of any particular order of colored rings increasing with decreasing particle size. Thus, aureole may be produced by relatively large particles that tightly compress the rings, or by the presence of a broad particle size distribution that causes the colored rings to overlap; corona by appropriately sized and uniform particles that provide a favorable angular separation of the colored rings; and iridescence by restricted groups of relatively small uniform particles, as near the edges of growing or evaporating clouds.

© 1990 Optical Society of America