Leaf-tobacco smoke coronas are well known to us -- but birch, pine and spruce coronas not so much! What gives? The corona here is not the cigar but ring of light produced by forward scattering of light from the sun or moon. This corona is usually due to a thin wisp of clouds between our eyes and the sun or moon. Light from these bodies heading our way is forward scattered by the tiny water droplets that comprise the cloud. Like rainbows, the light we see from the corona takes the form of colored rings. Spherical drops give circular rings. If the cloud above is littered with ice crystals, the rings may consist of dots of light or arcs of light above and below and left and right of the sun or moon. They are still nice circular features and, like a rainbow, there are sometimes pop up in multiples. As many as three nested corona rings have been observed. The higher up in the air the water drops are, the bigger the rings.
Pekka Parvianinen and Veikko Mäkelä observed coronas on cloud-free days in their native Finland. These observed corona were not circles but ellipses with the long axis in the vertical direction. Actually, they detected two different kinds of elliptical corona. One had nice uniform ellipses of forward scattered light at all heights above the horizon. The second type was elliptical, especially when the sun was near the horizon. The Finns have many days in the year with this ideal low sun angle. This second type of corona has bright spots at the top and bottom and on the left and right of the corona. It was as though there was ice in the air and the crystals were preferentially oriented. Colors? There were three bands. The inner-most zone was red to orange. The middle zone violet to red. Outermost zone of the corona was blue to red.
It may well be that these sharp eyed Fins also had a nose for the "hay fever" because the recorded in their notes the sharp peaks in air borne pollen (birch and pine) on the days of the sightings of the elliptical coronas. So they came up with the birch-pine-spruce, elliptical-clear-sky corona theory. They must have known that all pollen is not spherical. They show nice sketches of the three types of pollen. A quick glance at the figure below will convince you that this is no Mickey Mouse problem.
Eberhard Tränkle and Bernd Mielke, well mathematically abled, wrote computer code to simulate corona from pollen grains of known morphology. Their sketches of the pollen grain are quite modern with their pine pollen equipped with dual "air bags." Their computerized virtual coronas of birch were vertically elliptical rings. The "air bag"-enabled pollen corona was vertically elliptical with bright spots top and bottom and left and right. With wonderful specificity, they also concluded that the pine pollen at the time of the corona was in a vertical orientation, with wings up and a small tilt! While spruce was not common in the part of Finland where the pollen corona were seen, spruce pollen was also simulated for corona morphology. Spruce pollen is about 1.5 times the size of the pine pollen studied. The corona of the winged spruce pollen was much like that of the winged pine pollen. Color saturation was less in spruce and the corona is less well separated from the solar disk.
Birch, pine and spruce pollen grain morphology. Average diameters of the body cell of each are 24, 52 and 75 micrometers. The production of coronas by these pollen grains requires that they have a orientation while in the air. The aerodynamic forms of each in falling mode account for their position relative to the downward direction.
It is nice to know that the fine light shows we see in the sky are sometimes the results of the anther ejecta. The next time your eyes water, you acquire some post nasal drip, and the sneeze urge overcomes you, search the sky for some beauty on these difficult days.
P. Parviainen, C. F. Bohren, and V. Mäkelä (1994). Vertical elliptical coronas caused by pollen. Applied Opitics 33(21):4548-45451.
E. Tränkle and B. Mielke (1994). Simulation and analysis of pollen coronas. Applied Optics 33(21):4552-4562.