The Forrest Mims YouTube video An Exquisitely Rare Pollen Corona shows a pair of rings closely circled around the Sun with rainbow colors; blue on the inside.

The first annulus is easy to see, the second is faint so I've played with the color and contrast to bring out the red part to contrast with the blue sky.

Question: What is an "exquisitely rare pollen corona"? How can pollen produce several tight rainbow circles around the Sun, and why pollen specifically?

Annotated screen shot from the Forrest Mims YouTube video An Exquisitely Rare Pollen Corona https://www.youtube.com/watch?v=5besA3kwJ60 Screen shot from the Forrest Mims YouTube video An Exquisitely Rare Pollen Corona https://www.youtube.com/watch?v=5besA3kwJ60

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    $\begingroup$ In fact, it's not "exquisitely rare": I see pollen coronas like this yearly since having first noticed one — in first weeks of June at 60°N 30°E. $\endgroup$ – Ruslan Jul 20 at 16:02

This is an interesting phenomenon that I haven't seen before. To start off with a discussion of coronas in general, they are fundamentally caused by diffraction, a form of optical scattering where the path of light from the sun is "bent" as it passes by the edge of a small particle. A simple illustration is case D in this example figure showing light scattering about a small cloud droplet:

Scattering of a radiation beam: processes of reflection (A), refraction (B), refraction and internal reflection (C), and diffraction (D), from http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap8.html

The other interactions shown include reflection and refraction. For diffraction, when multiple interactions take place, the diffracted light produces interference patterns. A simple example is shown here for light being diffracted from two sides of a particle:

Illustration of how two points on a droplet surface can scatter light and act as sources of outgoing spherical waves, from https://www.atoptics.co.uk/droplets/corform.htm

Ultimately, a corona is the result of a great many of these interactions taking place in the presence of a very large number of particles. However, as to pollen coronas in particular, it turns out that the type of particle largely doesn't matter - diffraction is such a major component compared to other optical scattering processes (e.g., reflection or refraction) that a corona may be formed whether the particles are liquid or solid: Corona formation, to a good approximation, needs no knowledge of the droplet interior because the surface scattered waves predominate. It could be water, ink or coal - the pattern is almost the same. It depends primarily on the droplet size, shape and the wavelength of the light.

Pollen, it turns out, sometimes happens to be in a similar size range to cloud droplets. With diameters ranging between 15 and 200 μm, smaller pollen particles can be of similar size as cloud droplets (~20 μm in diameter), allowing them to have similar diffraction properties. In the end, the main difference is that pollen are less uniformly shaped than cloud droplets; preferential orientation as they are suspended in the air can cause the resultant corona to become elongated vertically or have particular bright patches in some cases.

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    $\begingroup$ Thanks for your answer! If you check the notes below the video it seems in this location there is a single species Juniperus asheii that’s producing most of the pollen on that day, and its pollen releases seems to be particularly heavy and problematic for folks with allergies. So in this case the pollen in the air may be quite monodisperse (have a narrow distribution in particle size), namely 19–22 μm in diameter. $\endgroup$ – uhoh Jul 19 at 22:29
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    $\begingroup$ Rainbows can occur in the desert - no rain required. A sandstorm can put enough particulate matter into the air to produce the same effect. This seems pretty similar. $\endgroup$ – Darrel Hoffman Jul 20 at 15:50

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