Sassen Kenneth
Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA.
Appl Opt. 2011 Oct 1;50(28):F1-5. doi: 10.1364/AO.50.0000F1.
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 ~27 μm 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 ~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.
直到最近人们才意识到，悬浮在大气中的树木花粉颗粒的散射可以产生日冕，而这些研究几乎都来自斯堪的纳维亚半岛。通过日冕摄影和地表花粉分析，本文表明，阿拉斯加内陆的纸桦树在五月中旬的北方绿化期会定期产生日冕。尽管这些平均直径约27μm的颗粒形状接近球形，但它们有三个参与萌发的表面突起，这些突起被认为有助于产生椭圆形日冕，而产生椭圆形日冕需要强烈的颗粒优先取向。对于太阳仰角约为35°-40°的观测，发现轴比约为1.2，平均半径为2.5°（对于二级红带）。由于特定形状的定向颗粒往往比随机定向的颗粒下落得慢，这种微观设计促进了花粉的横向传播，增加了树木的繁殖机会，这对先锋物种来说是一个特别重要的特征。
