corona
1. Coloured rings of lights, typically from blue inside to red outside, that sometimes appear to surround the Sun or Moon. The effect is created by diffraction of light by spherical water drops in such clouds as altocumulus. Compare HALO.

2. Concentric zones of one or more minerals surrounding a core mineral. Coronas can be formed in a number of ways. (a) The discontinuous reaction of minerals with a magma can be preserved as coronas around the original high-temperature mineral if the cooling rate is fast enough to prevent the reactions going to completion. (b) Late-stage fluids may react with an earlier primary mineral to develop a corona of secondary minerals. (c) Two minerals may undergo sub-solidus reactions (reactions occurring after the rock has solidified) to maintain equilibrium as a rock mass cools, developing a corona of lower-temperature minerals. These types of texture are also known as ‘reaction rims’. See also CRYSTAL ZONING.

3. One of the large, circular features (150–600km diameter) of uncertain origin, comprised of up to 10–12 subconcentric ridges and grooves, which surround an inner region of irregular relief, found on the surface of Venus, mainly in a latitudinal belt 55° N–80° N along the borders of Ishtar and Tethus Regio. Most are associated with what appear to be lava flows.

co·ro·na¹ / kəˈrōnə/ • n. (pl. -nae / -nē; -nī/ ) 1. Astron. the rarefied gaseous envelope of the sun and other stars. ∎  (also corona discharge) Physics the glow around a conductor at high potential. ∎  a small circle of light seen around the sun or moon, due to diffraction by water droplets.2. Anat. a crown or crownlike structure. ∎ Bot. the cup-shaped or trumpet-shaped outgrowth at the center of a daffodil or narcissus flower.3. a circular chandelier in a church.4. Archit. a part of a cornice having a broad vertical face.co·ro·na² • n. a long, straight-sided cigar.

corona (pl. coronae).
1. Part of a Classical cornice, called larmier, above the bedmoulding and below the cymatium, with a broad vertical face, usually of considerable projection, with its underside recessed and forming a drip protecting the frieze under it.

2. Circlet or hoop hanging from above, as over an altar: usually carrying candles, it is called a corona lucis, a good example of which survives in Aachen Cathedral.

corona Coloured rings of lights, typically from blue inside to red outside, which sometimes appear to surround the Sun or Moon. The effect is created by the diffraction of light by spherical water drops in such clouds as altocumulus. Compare halo.

corona Outermost layer of the Sun's atmosphere, extending for many millions of kilometres into space. The corona emits strongly in the X-ray region, and has been studied by X-ray satellites. The corona has a temperature of 1–2 million K.