Simultaneous ground-based photometric measurements of auroral O I(6300 ¿), O I(7774 ¿), O I(8446 ¿), and N2 + (4278 ¿) 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 ¿) and O I(8446 ¿) multiplets, which are not quenched at auroral altitudes. The observed relation between the intensities of the O I(6300 ¿) and O I(8446 ¿) emissions suggests that the dominant mechanism for the production of O I(8446 ¿) is electron impact excitation of atomic oxygen. An additional excitation mechanism for O I(774 ¿) 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 ¿) intensity.