An Etymological Dictionary of Astronomy and Astrophysics

The spectrum of the solar → chromosphere obtained during a → solar eclipse in the instant before or after → totality. In the flash
spectrum the usual solar → absorption lines are replaced with
bright → emission lines. This is because in that very short interval only the → photosphere is eclipsed by the Moon, and not the chromosphere. The American astronomer Charles A. Young was the first to observe it during the the solar eclipse of 1870 (December 22) in Spain.

See also: The flash designation comes from the fact that the change from dark-line to bright-line spectrum is very rapid; → flash; → spectrum.