An imaging Fabry-Perot interferometer for twilight OI 844.6 nm observations

Ground-based measurements of twilight airglow provide a relatively inexpensive method for monitoring thermospheric composition and photochemistry. An imaging Fabry-Perot spectrometer for twilight observation was developed and used for measuring the atomic oxygen multiplet emission at 844.6 nm during the fall of 1991 in evening twilight from the Toronto Airglow Observatory (TAO) in Toronto. PRESTO, a PRogrammable Etalon Spectrometer for Twilight Observations, previously was used in a piezo-electric etalon scanning mode but was re-designed for spatial spectral scanning which produced a time resolution improvement of 20 times.; Four twilights were selected from two months of observation during the fall of 1991. A least square method was developed and applied to separate the line emission from the tropopherically scattered white light background. The column emission rate as a function of solar zenith angle (SZA) was plotted with the appropriate correction for tropospheric transmittance. The spectral line average intensity was 800 Rayleigh (R) at 95{dollar}spcirc{dollar} SZA with the emission observable from 94{dollar}spcirc{dollar} SZA to 104{dollar}spcirc{dollar} SZA.; A review of the proposed mechanisms that are responsible for the emission concluded that it can be fully attributed to the photoelectron impact excitation of atomic oxygen. The field line interhemispheric plasma (FLIP) model developed by Richards et al. (1983) was employed to compute theoretical 844.6 nm intensity profiles for the observed conditions. A comparison of the FLIP model with the experimental data shows a close agreement for September and 50% over-prediction for October. Possible reasons for the over-prediction are discussed.