A rare solar halo display

While my wife, Deborah, and I were traveling in Nepal, our housekeeper, Shirley Shebadieta, was at our house in Maun, Botswana, capturing an unusual optical phenomenon with her smartphone on October 24, 2018. Shirley was outside hanging clothes when she noticed clouds approaching and then covering the Sun, producing a stunning array of odd-radius halo arcs.

Shirley said she was impressed with the intensity of the darkness within the circle, saying that at times it looked like an approaching storm. Shirley’s best images show a series of three equally spaced (or near-equally spaced) halo arcs well above the Sun and two arcs well below, with another 9°-wide halo centered on the Sun.

Shirley sent me the images, and when I returned home I borrowed her phone and took images of the Sun in a blue sky, to rule out camera defects and internal reflections as possible causes for the radius halos. But the abundance of images that Shirley took at different angles supported the reality of the sighting. Having never seen anything like this, I sent the images to Les Cowley, who runs the website Atmospheric Optics, and asked for his help to explain the phenomenon.

Les said the display was real but “very strange.” Indeed, after playing with ray-tracing simulations using HaloSim, he found the initial results problematic — until he set the Sun’s altitude to 80° and let the machine run out “millions of rays through the [ice] crystals.” And, voila! “On the screen, Shirley’s display popped straight out,” he said. The results showed the 9° halo, plus 20°, 22°, and 24° halo arcs above the Sun, and 22° and 24° halo arcs below the Sun.

The next morning, I awoke to an email from Les saying, “There is an explanation of the unusual sighting — but I need the altitude 80° or more.” Once again, Shirley lent me her phone. The most unusual displays began at 11:41 a.m. local time, when the Sun’s altitude was 79°15′. Les was ecstatic.

Ordinarily, Les explained, odd-radius halo displays are created by randomly oriented pyramidal crystals, but they do not explain these halos. “However,” he said, “put the Sun very high and pyramidal crystals oriented with their long axes roughly horizontal,” and we can reproduce Shirley’s images. A full analysis can be seen at www.atoptics.co.uk/opod.htm.

Given that Shirley is not a trained observer, she deserves praise for making a drawing based on her naked-eye view, as well as taking some 40 images of the phenomenon from different angles. “When something unusual is detected in the sky, always take several images,” Les advises.

By the way, the odd-radius halos have names after the observers who first reported them: The 9° halo is known as Van Buijsen’s halo; the 20° one is Burney’s; and the 24° halo is Dutheil’s. While these halos are among the most common of the odd-radius halos, they are rarely seen by one person. Meteorologist Alister Ling, now retired from Environment Canada in Edmonton, Alberta, saw his first odd-radius halo display (including the 9° halo) in the spring of 2018, noting it “only took 40 years of skywatching” to spy one.

As always, send your thoughts and observations to sjomeara31@gmail.com.