O I (7774 Ă) and O I (8446 Ă) emissions in aurora

Simultaneous ground‐based photometric measurements of auroral O I (6300 Ă), O I (7774 Ă), O I (8446 Ă), and N 2 + (4278 Ă) emissions and model calculations of their intensities are presented. Detailed examination of the variations in the observed brightness of these emissions indicates a decrease in the I (8446)/ I (7774) ratio as the red ( I (6300)) to blue ( I (4278)) ratio becomes smaller. This is contrary to the constant I (8446)/ I (7774) ratio expected from electron impact excitation of the O I (7774 Ă) and O I (8446 Ă) multiplets, which are not quenched at auroral altitudes. The observed relation between the intensities of the O I (6300 Ă) and O I (8446 Ă) emissions suggests that the dominant mechanism for the production of O I (8446 Ă) is electron impact excitation of atomic oxygen. An additional excitation mechanism for O I (7774 Ă) operating at relatively low altitudes in aurora is required to explain both the measured changes in the I (8446)/ I (7774) ratio and the O I (7774 Ă) intensity.