It has only recently been realized that solar corona can be generated by dispersions of tree pollen grains suspended in the atmosphere, and these studies have come almost exclusively from Scandinavia. Using corona photographic and surface pollen analyses, it is shown here that paper birch trees in the interior of Alaska regularly generate solar corona during the boreal green-out in mid-May. Although near-spherical in shape, these $\sim 27\mathrm{\mu m}$ average diameter particles have three surface protrusions involved in germination that are indicated to aid in the generation of elliptical corona, for which a strong preferential particle orientation is needed. For observations at solar elevation angles of $\sim 35°–40°$ , an axis ratio of about 1.2 and average radius of $2.5°$ (for the second-order red band) are found. Because oriented particles of a particular shape tend to fall slower than randomly oriented ones, this microdesign promotes the lateral spread of pollen and enhances tree reproductive opportunities, an especially important trait for pioneering species.

© 2011 Optical Society of America

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