Corona (optical phenomenon)

In meteorology, a corona (plural coronae) is an optical phenomenon produced by the diffraction of sunlight or moonlight (or, occasionally, bright starlight or planetlight)^[1] by individual small water droplets and sometimes tiny ice crystals of a cloud or on a foggy glass surface. In its full form, a corona consists of several concentric, pastel-colored rings around the celestial object and a central bright area called an aureole.^[2][3] The aureole is often (especially in case of the Moon) the only visible part of the corona and has the appearance of a bluish-white disk which fades to reddish-brown towards the edge. The angular diameter of a corona depends on the sizes of the water droplets involved; smaller droplets produce larger coronae. For the same reason, the corona is the most pronounced when the size of the droplets is most uniform. Coronae differ from halos in that the latter are formed by refraction (rather than diffraction) from comparatively large rather than small ice crystals.

Lunar corona

A solar corona up Beinn Mhòr (South Uist)

The corona intensity pattern resembles an Airy disk, since the diffraction pattern of a round obstacle is identical to that of a round aperture of the same diameter, except for the intensity of the undiffracted light, according to Babinet's principle.^[4]

Pollen coronae

Pollen suspended in the air can also cause diffraction of sunlight that produces coronae. Because pollen grains are not always spherical, the resulting pollen coronae often have characteristic elliptic shape and brighter spots in them. They can be seen during blooming season where there is significant source of pollen like forests. They are more easily seen during sunset or sunrise as there is less sun glare and the light path through pollen laden atmosphere is longer.^[5]

See also

• Atmospheric diffraction
• Glory (optical phenomenon)
• 22° halo