Pollen Corona
Imaged in Finland by Tom Eklund (skyscapes).
 ©Tom Eklund, shown with permission

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The pollen corona is recognisable by a definite ring outside the bright central aureole around the sun and the brightening at the ring’s top.    There is a hint too of a second outer ring.    Care is needed in interpretation because digital cameras can give corona-like artifacts around the setting sun.

Pollen particles scatter light waves from their surface.   Each point on the surface is a source of outgoing spherical waves.    The waves interact with each other.   In directions where wave crests coincide there is bright light.   Where the crests cancel there is darkness.   The result in the sky from millions of pollen grains is a circular diffraction or interference pattern – a corona.

Pollen grains are non-spherical and become aerodynamically oriented as they drift in the wind.   The oriented non-spherical shape causes their coronae to be oval with bright spots on the rings.