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Sun Corona imaged by Kevin Boyle near Stoke-on-Trent, Northern England. Image ©Kevin Boyle, shown with permission.

"The image was taken 13th September with an off the shelf digital camera. The corona was caused by a fairly large altocumulus lenticular cloud, a common source for this kind of phenomenon."

Coronae are produced when light is diffracted by the small droplets of clouds. This one is exceptional in that three rings are visible, indicating that the droplets were all of very similar size. Droplets form and then evaporate rapidly in lenticular clouds, a factor producing similar drop sizes because all the droplets have identical histories.

The image is also exceptional in that the sun's disk is visible. Usually it is lost in overexposure. The disk, 0.5° diameter, lets us measure the corona angular size and then estimate the size of the droplets making it. The inset image compares the corona with a simulation made by an exact Lorenz-Mie calculation in IRIS. The corona is reasonably matched (its rings are slightly non-circular) using a droplet diameter of 15 micron (15/1000 mm). This is well within the 1-100 micron diameter range of cloud droplets.