The top corona is almost circular. The central aureole tapers in colour from straws to reds. The surrounding ring is not spectral in its colour range for each hue is a colour mixture. The ring is slightly flattened at its top.
At right is a more pathological case.
Coronae are diffraction patterns in the sky produced by sunlight scattered from millions of small water droplets. The diffraction is from individual water droplets acting in isolation and definitely not collectively like a diffraction grating as claimed in this erroneous university website.
The smaller the droplets the greater the angular size of the corona. When the droplet diameter varies in a fairly orderly manner across a cloud the corona is distorted. At right the droplets in the lower right corner are the smallest.
A less orderly spatial arrangement of droplets sizes gives colours tending to the chaotic - cloud iridescence.
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