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Corona, city, United States
corona, in astronomy
Optically, the solar corona has two main components: the K-corona (or inner corona) consists of rapidly moving free electrons, exhibits a continuous spectrum, and attains a temperature of around 2 000 000 K at a height of about 75 000 km; the F-corona (or outer corona) consists of relatively slow-moving particles of interplanetary dust, exhibits an absorption spectrum, and extends for many million kilometers into the interplanetary medium. A third component, the E-corona, consists of relatively slow-moving ions and exhibits an emission spectrum superimposed on the continuum of the K-corona.
The white-light corona comprises the overlapping K-corona and F-corona. It is visible under natural circumstances only in profile beyond the Sun's limb on the rare occasions when the photosphere is totally eclipsed by the Moon. It may, however, be observed out to distances of several solar radii at times other than totality, with the aid of a balloon- or satellite-borne externally occulted coronagraph. Similarly, the E-corona may be observed at specific wavelengths with the aid of the Lyot coronagraph, used at certain high-altitude observatories.
The corona may also be observed against the Sun's disk at extreme-ultraviolet and X-ray wavelengths, using rocket- or satellite-borne instrumentation. X-ray observations reveal the structure of the corona at temperatures of several million kelvin. Strong X-ray emission is associated with active regions, and an absence of X-ray emission with coronal holes. There is little or no evidence for a uniform corona – its structure is determined by the strength and configuration of the localized magnetic fields. The X-ray telescopes on Skylab and the Solar Maximum Mission, in particular, provided much additional information on coronal structure (see coronal transients).
The overall shape of the solar corona changes with the phase of the sunspot cycle. At sunspot minimum it is roughly symmetrical, with long equatorial streamers, and plumes orientated in the direction of the Sun's polar magnetic field. At sunspot maximum it is less symmetrical, although more evenly distributed about the Sun's disk as a whole. Its changing shape is due principally to the presence of individual streamers above active regions, the mean heliographic latitude of which progresses toward the equator as the sunspot cycle proceeds. Solar radio emission at meter wavelengths originates in the corona and may exhibit an intense burst or series of bursts at the time of a large flare.
It has been found that in general the coronae of normal stars are sources of X-rays: the X-ray telescope on the Einstein Observatory was able to detect all types of stars apart from red supergiants. The idea that coronae are heated by shock waves rising through the chromosphere from the photosphere (acoustic heating) is now considered untenable, because such heating would occur only in late-type stars like the Sun. Instead, the coronal output is thought to be linked to the star's rotation. In certain binary systems (RS Canum Venaticorum stars and some flare stars) the orbital motion forces the stars to spin rapidly, and they are unusually strong X-ray sources. Young stars, such as those in the Orion nebula or the Pleiades, rotate faster than older stars of the same spectral type; they too have powerful X-ray emitting coronae. The link is almost certainly that fast rotation considerably enhances the magnetic field, and this provides greater heat input to the corona; the details, however, are still obscure.
light, misty rings in the heavens around the disk of the sun or moon and, less frequently, around bright stars and terrestrial light sources. A corona forms when translucent clouds (usually altocumulus) or fog passes in front of the light source; a corona is distinguished from a halo by the former’s smaller radius (not more than 5°). Coronas are explained by the diffraction scattering of rays from the light source on the drops of water that form the cloud or fog. On the basis of the theory of coronas, it is possible to determine the diameter of the drops in clouds according to the observed angular radii of individual rings or the dark spaces that separate them and to discern clouds consisting of water and ice particles. The radii of the rings depend on the wavelength of the rays which cause the coloration of the corona; the outermost ring takes on a reddish color. When drops of all possible sizes are present in the atmosphere, the superimposed corona rings form a general white glow around the disk of the light source; this is called a perihelion aureole.
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.
corona(1) See corona wire.
(2) (Corona) The code name for Windows Media Player 9, introduced in 2003. It improved the Windows Media Video codec and added the Windows Media Audio Professional codec for 5.1 surround sound at 24 bit, 96 kHz sampling. It also included Fast Stream, providing instant-on audio and video streaming for high-bandwidth connections.