corona

i. Rims of small radius and of subdued color commonly observed surrounding, and sometimes apparently touching, the solar disc. They are caused by the diffraction of sunlight or moonlight through thin-veiled clouds. When discernible, the colors of shorter wavelengths—blue and green—appear on the inner portion of the corona, with red and orange forming the outer ring—an arrangement just opposite to that in a halo. Coronas vary in size and are much smaller than the fixed size of a halo. Coronas form in clouds, which are composed of water droplets, and halos form in cirriform clouds, which are composed of ice crystals. See also broken specter.

ii. The blue or purple glow that is visible when the voltage on an electrical conductor, such as an antenna or a high-voltage transmission line, exceeds a certain value. This causes ionization of the air around the conductor and the resultant glow.