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POLLEN CORONAS IN FOCUS 2/3
(2) Even here in Finland we have observed mainly three type of
coronas: birch, pine and spruce, there must be several other
plants, mainly threes, which can release enough pollen grains
to form coronas. We expect at least alder and willow pollen.
There must be rather high density of pollen in air, before
coronas appear and mainly strongly flowering trees can produce
good pollen cloud. There is no need that parent trees are
very near the observer, because in good conditions pollen
grains can fly hundreds of kilometers by winds.
The pollen coronas is a such nice phenomenon, that you can conclude
the approximate shape of grains from the shape of corona. If
the grain is spherical, also the coronae is spherical. If the
grain is elliptical, the coronas are oval. In this case the
orientation of grain and coronae ellipse is in turned 90
degrees. Eg. if the grains are lying horizontal in the air, we see
vertical elliptical coronas. The physical reason have explained
by Bohren in above mentioned Applied Optics article. The shape
of ellipse varies by solar altitude. That is due to viewing
direction. We see only the projection of ellictical shape of
pollen grain. And further, the diameter of coronae gives an
approximately size of particle. By the way, is there any
simple formula to calculate corona size from particle size.
At least it is inversely propotional the size. As bigger
grains as smaller coronas.
A symmetric grains gives symmetric coronas. But often pollen
particles are not symmetric. Eg. conifer trees have often
two air bags in the main grain, so that they look like "Mickey
Mouses". Any asymmetricities produce complicated features
to coronae, such as brightening on the top and sides of corona.
These are very difficult to calculate analytic. Traenkle and
Mielke from Free University of Berlin have made computer
simulations (Traenkle and Mielke: Simulation and analysis
of pollen coronas, Appl. Opt. Vol 33 No. 21, 20 July 1994,
pp. 4552-4561).
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