An Etymological Dictionary of Astronomy and Astrophysics

1. A prominent → forbidden line in the spectra of the aurora at wavelengths of 5577 and 6300 Å giving the aurora its characteristic green and red colors.
   Both are emitted by atomic oxygen, the former in its transition from the second lowest excited electronic state (¹S) to the lowest excited electronic state (¹D), the latter in its transition from the lowest excited electronic state (¹D) to the atomic ground state (³P).
   
2. A forbidden line emitted by interstellar ionized gas by several atomic species (O, O⁺, O⁺⁺, N⁺, S⁺⁺, etc.) corresponding to the transition from the state ¹S to ¹D. → forbidden line; → nebular line; → transauroral line.

See also: → auroral; → line.