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Definition:
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The sporadic radiant emission from the upper atmosphere over the middle and high latitudes. It is believed to be due primarily to the emission of the nitrogen molecule N2, its molecular ion N2+, and atomic oxygen [O]. According to various theories, auroras seem definitely to be related to magnetic storms and the influx of charged particles from the sun. The exact details of the nature of the mechanisms involved are still being investigated. The aurora is most intense at times of magnetic storms (when it is also observed farthest equatorward) and shows a periodicity related to the sun's 27-day rotation period and the 11-year sunspot cycle. The distribution with height shows a pronounced maximum near 100 km. The lower limit is probably near 80 km. The aurora can often be clearly seen, and it assumes a variety of shapes and colors that are characteristic patterns of auroral emission. The names given to the various forms are 1) arcs, which are bands of light extending across the sky, the highest point of the arc being in the direction of the magnetic meridian; 2) rays, which may appear as single lines like a searchlight beam, or in bundles; 3) draperies, which have a curtainlike appearance, sharp on the bottom and tenuous in the upper parts; 4) crown or corona, which are seen when the rays appear to spread out from a single point in the sky; 5) bands, which are similar to the arcs, and may or may not have a ray structure; and 6) diffuse luminous surfaces, which appear as luminous clouds of indefinite shape. Sometimes the term "streamers" is used to describe the auroral forms that extend to great heights. In northern latitudes these displays are called aurora borealis, aurora polaris, or northern lights; in southern latitudes they are called aurora australis.
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