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Published online by Cambridge University Press: 06 September 2019
As is well known, the word “chromosphere” was coined to denote the bright, thin, colored ring seen as the solar limb was obscured by the Moon at the time of a total eclipse. This region of the Sun's atmosphere was found to be the source of many strong emission lines — the flash spectrum — some persisting to such heights as to leave no doubt that their cores originated quite high in the chromosphere. The presence of such an emission line region is not unexpected; however, what gives the solar chromosphere special interest is the fact that its observed spectroscopic properties cannot be explained on the basis that it is a simple extension of a “classical” atmosphere for which radiative, hydrostatic, and local thermodynamic equilibrium all apply Thus, the height above the limb to which most eclipse lines persist is inconsistent with the predicted density scale height.