Evening twilight spectra of the OI 8446 ¿ emission were obtained during May and June of 1993 using a single-etalon, pressure scanning, Fabry-Perot interferometer located in the Millstone Hill Optical Facility. The goals of this work are to positively identify the 8446 ¿ emission in the twilight airglow and to determine the intensity decay as a function of solar depression angle. Also, a study of the relative triplet line strengths is performed in hopes of establishing the importance of the primary excitaiton mechanisms (photoelectron impact or Bowen fluorescence) during the twilight period. Although absent in most of the data, a distant auroral influence is also found to contribute considerably, on occasion, to the emission over Millstone Hill. The ratio of the combined 8446.26 ¿ and 8446.38 ¿ intensities to the 8446.76 ¿ intensity varies as 0.13¿0.03 per degree of solar depression angle, indicating that secondary excitation mechanisms are becoming increasing important as evening twilight progresses.

Bowen fluorescence is not found to be the primary excitation mechanism at any time during twilight, contributing just a few Rayleghs at most. These observations are an important first step toward a better characterization of highly variable thermospheric oxygen concentrations through ground-based measurements of the OI 8446 ¿ emission. ¿ American Geophysical Union 1994