A Tale of Two Pollens

An unusual pollen corona imaged in Finland by Jari Luomanen (optics images). The oval rings are characteristic of pollen coronae. However, there are too many rings! There are actually two superimposed coronae made separately by spruce and birch pollen grains.

©Jari Luomanen, shown with permission.
Atmospheric
Optics
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A pollen grain with air sacs scatters incoming plane parallel sunlight waves from its periphery.

Outgoing spherical waves overlap and interfere to produce a (far field) diffraction pattern.

Millions of grains give the diffraction pattern in the sky.

A corona – a bright aureole around the sun or moon and surrounded by one or more coloured rings – is generated by diffraction.   Most coronae - from small water drops or cloud ice crystals - are circular.

Pollen grains are different, they are non-spherical and a good many have two air sacs to help them carry in the wind. The air sacs quite incidentally orient the grains and these produce oval coronae sometimes with bright spots.   

Corona size depends on pollen size – small grains give larger coronae.   The small green grains in Jari Luomanen’s sample at right are birch pollen. They produced the larger coronal rings.   The much larger white spruce pollen produced the smaller inner corona.