1. The outer part of the sun’s atmosphere. Its two main components are the K-corona (or inner corona), with a temperature of about 2×106 K at a height of some 75 000 km, and the F-corona (or outer corona), which is considerably cooler and extends for several million kilometres into space.
2. A glowing region of the air surrounding a conductor when the potential gradient near it exceeds a critical value. It is caused by ionization of the air and may be accompanied by hissing sounds. Corona discharge (or point discharge) occurs at sharp points where the surface charge density is high by the attraction, charging, and consequent repulsion of air molecules.
1. An extremely hot (on average about 2 million K), highly ionized but tenuous gas surrounding the Sun. Certain other stars also have coronae. The Sun’s corona is visible at total eclipses as a white halo extending out to several solar radii, displaying streamers, plumes, and loops. The radiation of the white-light corona has components due to emission lines (the E corona) along with scattering from electrons (the K corona) and dust particles (the F corona). The corona’s outermost extension is the solar wind, which flows into interplanetary space.
Emission lines at X-ray and extreme ultraviolet wavelengths due to highly ionized atoms (coronal lines) indicate that the temperature of the corona is millions of degrees, the highest temperatures of 10 million K or more being found in active regions. Magnetic fields of about 10−3 tesla permeate the corona, governing its shape and structure. In active regions the magnetic fields form closed loops, but in coronal holes the magnetic field lines are open and stretch out into space.
The energy that heats the corona is thought to come from motions in and below the solar photosphere. There are two possible methods for releasing the energy. In the wave-heating scenario, high-frequency photospheric motions create waves in the Sun’s magnetic field that travel outwards into the corona, where they dissipate. Conversion of the wave energy into heat could account for the high temperatures of the coronal gas. An alternative heating mechanism requires low-frequency photospheric motions to tangle up the magnetic lines of force in the corona. Eventually the stresses in the magnetic field are released through a process termed magnetic reconnection of field lines. The energy released in such reconnection events is expected to vary widely, from flares to so-called microflares and even smaller nanoflares.
The appearance of the corona changes during the solar cycle. At solar maximum it consists of many active-region loops and streamers around the disk, but at solar minimum it is dominated by large coronal holes at each pole and a sheet-like structure near the equator.
Main-sequence stars cooler than spectral type F0 often have coronae with active regions, as indicated by their X-ray emission. This is particularly true of the M-type dwarf flare stars. Coronae are also present in some interacting binary systems like the RS Canum Venaticorum stars.
2. A large circular or elongated feature on a planetary surface, surrounded by concentric ridges; pl. coronae. The name, which means ‘crown’ or ‘circle’, is not a geological term but is used in the nomenclature of individual features, for example Nightingale Corona on Venus and Arden Corona on Uranus’s satellite Miranda.
3. A region of tenuous, very hot gas extending out of the galactic plane in spiral galaxies such as the Milky Way; also called galactic corona.
http://www.eso.org/public/images/eso0127a/
The faint halo of hot (about 2 000 000° C) and tenuous gas around the Sun, which boils from the surface. It is visible at solar eclipses or through a coronagraph, an instrument that blocks light from the Sun's brilliant disc. Gas flows away from the corona to form the solar wind. NASA's Reaven Ramaty High-Energy Solar Spectroscopic Imager mission was launched in 2001 to study the evolution of energy in the corona.
1. White or occasionally 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 (150–600 km diameter) of uncertain origin, comprising 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 between 55° N and 80° N along the borders of Ishtar and Tethus Regio. Most are associated with what appear to be lava flows.