Scientists discover why the corona is as hot as it is

Scientists crack 70-year-old mystery of how magnetic waves heat the sun

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This explains why the corona of the sun is so much hotter than its surface.

Corona

A corona (Latin, 'crown') is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word corona is a Latin word meaning "crown", from the Ancient Greek ?????? (koronè, “garland, wreath”).

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". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV, or Fe13+). 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 Fe13+ at 5303 Å, but also the red Fe-X line from Fe9+ at 6374 Å). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvins,[1] much hotter than the surface of the Sun.