In atmospheric optics, a corona is a set of one or more small colored rings of scattered light surrounding the sun, moon, or other light source. It should not be confused with the sun’s outer atmosphere that goes by the same name. As shown in Fig. 2.19, a corona appears near the 0° scattering angle. Observing these colors requires looking close to the sun or moon, so care must be taken to protect your eyes by blocking the direct sun. Coronas are easier to see around the moon (“lunar coronas”) because the moon is not such an overwhelmingly bright object.
Corona rings are typically much smaller than halo rings (Fig. 1.1). Laven1 used Mie scattering simulations to show that the first three red corona rings have angular radii in degrees given by 16/r, 31/r, and 47/r, where r is the radius of the scattering particles in m. The largest corona rings are formed by the smallest cloud droplets, so typical cloud droplets with a radius of 10 m would generate corona rings with angular radii of 1.6°, 3.1°, and 4.7°. However, it is quite common to see only the first ring or two. It is a visual treat to see three or more concentric rings.
The conventional explanation says that a corona is formed by light diffracted by spherical water droplets in a cloud. Diffraction causes light incident on a circular object or aperture to spread into a concentric pattern of rings surrounding the light source. Since diffraction is just near-forward scattering, Mie scattering provides a more accurate model of the corona. However, insights have been gained using the simple equations for scalar diffraction from a circular obstruction. The cloud must be partially transparent so the light passes through the cloud and reaches the observer. All of the droplets must also have a similar size because different particle sizes generate different ring sizes that overlap and smear the colors into a single white ring.
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