Spectral Contamination of the 6300 Å Emission in Single-Etalon Fabry-Perot Interferometers

The spectral line profile of the atomic oxygen O¹D₂—³P₂ transition near 6300 Å in the airglow has been used for more than 50 years to extract neutral wind and temperature information from the F-region ionosphere. A new spectral model and recent samples of this airglow emission in the presence of the nearby lambda-doubled OH Meinel (9-3) P₂(2.5) emission lines underscores earlier cautions that OH can significantly distort the OI line center position and line width observed using a single-etalon Fabry-Perot interferometer (FPI). The consequence of these profile distortions in terms of the emission profile line width and Doppler position is a strong function of the selected etalon plate spacing. Single-etalon Fabry-Perot interferometers placed in the field for thermospheric measurements have widely varying etalon spacings, so that systematic wind biases caused by the OH line positions differ between instruments, complicating comparisons between sites. Based on the best current determinations of the OH and O¹D line positions, the ideal gap for a single-etalon FPI wind measurements places the OH emissions in the wings of the O¹D spectral line profile. Optical systems that can accommodate prefilters with square passbands less than ∼3 Å in the optical beam can effectively block the OH contamination. When that is not possible, a method to fit for OH contamination and remove it in the spectral background of an active Fabry-Perot system is evaluated.