Pollen Corona

Last updated on September 16, 2023

Pollen Corona: Exploring the Phenomenon of Multi-Ringed Optical Delight

When we gaze at the sky, we often witness captivating atmospheric phenomena that leave us in awe. One such spectacle is the pollen corona, an optical phenomenon created by the scattering of sunlight by pollen grains suspended in the air. While the concept of a corona is commonly associated with rainbows, fogbows, and glories, the pollen corona stands out as a unique phenomenon that doesn't rely on transparent particles. In this article, we will delve into the fascinating world of pollen coronae and explore the intricacies of their formation.

The Characteristics of a Pollen Corona

A pollen corona is distinguishable by its slightly vertically elongated and multi-ringed appearance. The multi-ringed structure arises from the fact that pollen grains, responsible for scattering sunlight, are uniform in size. These grains, which possess air sacs resembling giant ears, facilitate their widespread dispersal by the wind. The air sacs orient each grain, resulting in the observed elongation and multi-ringed pattern of the corona. To visualize this, imagine looking upwards at the underside of a pollen grain, with the air sacs extending at each side.

Unraveling the Size and Shape of Pollen Grains

Determining the size of pollen grains is no easy task, but innovative techniques shed light on their dimensions. By analyzing camera and lens data, scientists can measure the horizontal diameter of the corona's first red ring, which provides an estimate of the pollen grain size. Interestingly, pollen coronae tend to be smaller than their water droplet counterparts. Through Mie scattering simulations conducted by IRIS, an effective spherical particle diameter of 35 microns was obtained for the pollen grains. This finding is remarkable considering the non-spherical nature of pollen.

The Influence of Sun's Altitude on Pollen Coronae

Observing pollen coronae at different times of the day reveals intriguing variations. As the sun descends, the effects of pollen non-sphericity become more apparent. For instance, when the sun's altitude was 36 degrees, an image captured a pollen corona with a slight elongation. However, two hours later, when the sun was only 19 degrees above the horizon, a more elongated corona with noticeable bumps on the first ring was observed. These changes in corona morphology highlight the dynamic nature of atmospheric optics and the role of solar position in shaping these phenomena.

The Mechanics of Pollen Corona Formation

Unlike rainbows, fogbows, and glories, pollen coronae do not rely on transparent particles. Instead, they arise from the direct scattering of light waves by the surface of the pollen grains. While transparent particles may contribute minorly through higher-order waves passing inside, their presence is not crucial to the formation of the corona. This distinction makes pollen coronae a unique and intriguing phenomenon within the realm of atmospheric optics.

Visualizing Pollen Grains in Detail

To gain a closer look at pollen grains responsible for creating the corona, scientists have utilized electron microscopy. These high-resolution images provide valuable insights into the intricate structures of pollen grains. In one such image captured by Gabija Bickauskaite at Vilnius University, two air-filled sacs can be observed on either side of the grain. These sacs aid in wind dispersion, allowing pollen grains to travel far and wide. The detailed visualization of pollen grains enhances our understanding of their unique characteristics and their role in creating stunning optical displays.

In Conclusion

Pollen coronae offer us a glimpse into the captivating world of atmospheric optics. Their multi-ringed structure and elongated appearance are a testament to the unique properties of pollen grains suspended in the air. By studying these phenomena, scientists continue to unravel the intricacies of light scattering and its interaction with particles in the atmosphere. The exploration of pollen coronae not only enriches our knowledge of optical phenomena but also serves as a reminder of the mesmerizing beauty that surrounds us in the natural world.

Pollen Corona

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.

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