Golub, Leon Harvard-Smithsonian Center for Astrophysics, Smithsonian Astrophysical Observatory, Cambridge, Massachusetts.
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The outer atmosphere of the Sun. The corona is characterized by extremely high temperatures of several million kelvins, which cause it to extend far above the denser surface regions of the Sun. Coronal gas is constrained to follow the magnetic field of the Sun, forming it into the shapes seen during a solar eclipse, with a coronagraph, or in x-rays. These shapes include long streamers that penetrate interplanetary space, looplike tubes over the strongest fields, and vast regions of very low density called coronal holes. Near the north and south poles, the coronal polar rays resemble the pattern formed by iron filings near a bar magnet, but at mid-latitudes, the more complicated fields of active regions dominate. The corona is hot enough to emit x-rays, and x-ray telescopes in space can form images of the corona. Such images display the magnetic loops connecting bright regions in the lower corona, while visible-light observations can detect the corona above the edge of the solar disk by carefully blocking out the strong photospheric light with an occulting disk. See also: Coronagraph; Eclipse; Solar magnetic field; X-ray astronomy
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