corona, luminous envelope surrounding the sun, outside the chromosphere. Its density is less than one billionth that of the earth's atmosphere. The corona is visible only at the time of totality during a total eclipse of the sun. It then appears as a halo of light with an irregular outer edge, often with streamers radiating from the sun's surface and contrasting with the dark lunar disk that it borders. It is divided into the inner corona, a ring of pale-yellow light against which crimson prominences are outlined, and the outer corona, a pearly white halo that extends far out into space. The corona consists of ionized gas at a temperature of 1 million°C;. By means of the coronagraph, the innermost part of the corona can be studied and photographed in full daylight. Although the visible corona extends a few solar radii above the sun, because of its high temperature it produces a continual flow of electrically charged particles called the solar wind that move outward through the solar system.

Corona Borealis [Lat.,=the northern crown], northern constellation lying between Hercules and Boötes. Its name derives from the crown Bacchus gave Ariadne when she was deserted by Theseus. The constellation is a small arc of bright stars, of which the brightest is Alphecca (Alpha Coronae Borealis). Corona Borealis reaches its highest point in the evening sky in early July.

Corona, city (1990 pop. 76,095), Riverside co., S Calif.; inc. 1896. The city developed as a primary citrus fruit producer and shipping center. There is also light manufacturing. The name Corona ("circle") was derived from the 3 mi (5 km) circular drive around the city that was once used for car racing. State prisons for men and women are nearby. Cleveland National Forest and hot springs lie in the vicinity.

Glow accompanying the brushlike discharges of atmospheric electricity that usually appears as a tip of light on the extremities of such pointed objects as church towers or the masts of ships during stormy weather. It is commonly accompanied by a crackling or fizzing noise. It is commonly observed on the periphery of propellers and along the wing tips, windshield, and nose of aircraft flying in dry snow, in ice crystals, or near thunderstorms. St. Elmo is an Italian corruption of St. Erasmus, patron saint of Mediterranean sailors, who traditionally regarded St. Elmo's fire as a sign of his guardianship over them.

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Outermost region of the Sun's (or any star's) atmosphere, consisting of plasma. The Sun's corona has a temperature of about 3.6 million °F (2 million °C) and a very low density. Extending more than 8 million mi (13 million km) from the photosphere, it has no definite boundaries, continually varying in size and shape as it is affected by the Sun's magnetic field. The solar wind is formed by expansion of coronal gases. Only about half as bright as the full moon, the corona is overwhelmed by the brilliance of the solar surface and normally not visible to the unaided eye, but a total eclipse permits naked-eye observations.

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