1. The outer visible envelope of the sun. Also called solar corona.
It is observed at solar eclipse or with the coronagraph. The shape of the corona varies during the sunspot cycle. At sunspot minimum the corona has large extensions along the sun's equator, with short brushlike tufts near the poles. At sunspot maximum the equatorial extensions are much smaller and the corona is more regular in shape. The temperature of the corona appears to be in the vicinity of 1,000,000 K.
2. The extremely tenuous outer atmosphere of the sun now known to extend past the earth's orbit.
3. A set of one or more prismatically colored rings of small radii, concentrically surrounding the disk of the sun, moon, or other luminary when veiled by a thin cloud.
The corona is due to diffraction by numerous water drops. It can be distinguished from the relatively common halo of 22 by the much smaller angular diameter of the corona, which is often only a few degrees, and by its color sequence, which is from blue inside to red outside, the reverse of that in the 22 halo.
4. See corona discharge.
5. See aurora.
6. See geocorona.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use