The corona is not always evenly distributed across the surface of the Sun. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux and hence generating the solar wind. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and magnetohydrodynamic waves from below. The corona is separated from the photosphere by the relatively shallow chromosphere. The corona is 10 −12 times as dense as the photosphere, and so produces about one-millionth as much visible light. The Sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the photosphere's average temperature is around 5 800 kelvin compared to the corona's 1 to 3 million kelvin. Physical features Ī drawing demonstrating the configuration of solar magnetic flux during the solar cycle
Bengt Edlén, following the work of Grotrian (1939), first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line from Fe 13+ at 5 303 Å, but also the red Fe-X line from Fe 9+ at 6 374 Å). Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV, or Fe 13+). The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, " coronium". In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny 'nanoflares', miniature brightenings resembling solar flares that would occur all over the surface of the Sun. In 1930, Bernard Lyot invented the coronograph, which allows viewing the corona without a total eclipse. French astronomer Jules Jenssen noted, after comparing his readings between the 18 eclipses, that the size and shape of the corona changes with the sunspot cycle. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, which was called helium. Based in his own observations of the 1806 solar eclipse at Kinderhook (New York), de Ferrer also proposed that the corona was part of the Sun and not of the Moon. In 1809, Spanish astronomer José Joaquín de Ferrer coined the term 'corona'. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun, not to the Moon. In 1724, French-Italian astronomer Giacomo F. Corona sketched by José Joaquín de Ferrer during the solar eclipse of Jin Kinderhook, New York.
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