Imaged by Audrius Dubietis of Vilnius University, Lithuania. ©Audrius Dubietis, shown with permission.
|The corona - from pine pollen - is characteristically slightly vertically elongated and multi-ringed.
Multi-ringed because the pollen grains scattering the sun’s light are all of the same size.
Slightly elongated because the pollen is non-spherical and specially oriented.
At right is a sample of the pollen creating the corona. The pollen has air sacs like two giant ears so that it is carried widely by the wind. The air sacs ride uppermost and orient each grain. Here we are looking ‘upwards’ at the underside of the grain and the air sacs are in the distance at each side. The main grain is ~30 micron (0.03mm) across.
There is another way to measure the pollen size. From the camera and lens data we get the corona's first red ring horizontal diameter as 3.4° (pollen coronae are usually smaller than their water droplet counterparts). A Mie scattering simulation by IRIS then gives an effective spherical particle diameter of 35 micron. Not bad agreement considering that the pollen is non-spherical.
Coronae show the effects of pollen non-sphericity more as the sun descends. The sun's altitude was 36° in the upper image. The lower image with a more elongated corona was taken two hours later when the sun was only 19° high. The lower corona also has noticeable bumps on the first ring.
Pollen coronae tell us that, unlike rainbows, fogbows and glories, coronae do not need transparent particles. The corona is produced by light waves scattered directly from the particle surface. When the particles are transparent there is a very minor contribution from higher-order waves that pass inside but it is not important.
|Gabija Bickauskaite at Vilnius University took this electron microscope image of the pollen creating the corona. At left and right are two air filled sacs that help the grains drift with the wind and disperse widely. The white bar is 30 micron long.|