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Coronae and Aureoles

Coronae are a diffraction phenomenon caused by small particles in the atmosphere like water droplets and ice crystals. There are solar and lunar coronae, the former having nothing to do with the Sun’s outer atmosphere. Coronae have a white centre and concentric coloured rings in an order that repeats itself up to four times (actually even more, but these rings are too faint to be observed). The simplest form of a corona is an aureole with a bright bluish centre and a single brownish ring. Coronae can show all colours with particularly vivid blues and greens.

There are no pure colours in coronae. Corona and iridescent colours more often than not have compoents from two or more quite different wavelengths resulting form the overlap of different diffraction orders. It is this that give iridescent colours their special electric or vibrant quality.

Coronae can occur around any bright light source, even around ice halos.

Please keep in mind that the strong light can cause eye damage, so never look directly at or near the Sun. Look for a place where the Sun is hidden by a tree or any other object or use your hand to hide it, always wear sunglasses. The light can also damage digital cameras, use a neutral density filter!
 
Simulations with IRIS by © Les Cowley shown with permission.


Droplet size = 8 miron


Droplet size = 16 micron

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