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A corona (not to be confused with the actual solar corona) is caused by diffraction of light by small cloud droplets, ice crystals, pollen or other particles. Due to the wave-nature of light, light is bend slightly after skimming the edge of a particle. By constructive and destructive interference of this light, diffracted over small angles by millions of small particles, colored rings can show up around the sun or the moon (and even the bright planets). The aureole is the first ring of the corona, consisting of a blue-white disk with the moon or sun at its center, bordered by a red-brown ring. The aureole is very common, when the sun or moon shines through a cloud deck. The actual corona, with sometimes many colored rings, is less common, because the cloud particles have to be of uniform size to allow for constructive interference of the light.
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