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In astronomy, the solar corona, the outermost layer of the solar atmosphere, which extends from an altitude of ~ 3000 km on the photosphere up to interplanetary space, where it merges with the solar wind. Because of their high temperature, the coronal hydrogen gas and helium, with smaller percentages of other elements are highly ionized, so that constitute a plasma. The crown, except during total eclipses of the sun, is invisible to the naked eye, because it is masked by the light, far more intense, emitted from the photosphere. The shape and the brightness of the crown vary during the cycle of solar activity: around the maximum of activity, the visible crown reaches its maximum extension and is approximately spherical, while in periods of low activity, the brightness is mainly concentrated in the equatorial belt. The end of the crown structure, especially in the polar areas, consists of thin filaments (rays), the shape of which suggests the presence of a dipolar magnetic field. The coronagraph is an instrument devised by B.-F. Lyot in 1930, which allows you to see and photograph the solar corona at all times in which the star is visible, no need to wait for the rare and short total solar eclipse. It essentially consists of a telescope, which brings in the plan focal lens a circular screen that perfectly hides the bright disk of the Sun; in this way the masked direct light from the solar photosphere, which otherwise would cover the faint light from the crown. Today, the emission of the crown is also being studied outside the visible band. While the observations at low frequencies (radio waves) can be conducted from the ground with radio telescopes, those at high frequencies must be carried out, to the outside atmosphere, on board of rockets or artificial satellites. The in XUV observations led to the discovery of two coronal structures: coronal holes and the bright points. The first, which appear as dark spots in the photographs in X-rays, are relatively cold areas of the crown, characterized by single-pole magnetic fields. The larger and more stable coronal holes tend to form in the polar regions. The bright points have a duration of about 8 hours and typical size of 20,000 km; some of them are associated with the development of the active regions.
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