Observation of the 557.7 nm to 297.2 nm brightness ratio in the auroral spectrum with OSIRIS on Odin

Abstract

The two optically forbidden lines of atomic oxygen, O(1S–1D) at 557.7 nm and O(1S–3P) at 297.2 nm, serve as important diagnostics in atmospheric, planetary, and cometary studies. Originating from the same upper state, the emission brightness ratio, B(557.7)/B(297.2), must necessarily be constant. The reported emission ratio of these two lines from both theoretical and experimental investigations varies by approximately a factor of two. These two emissions are observed simultaneously in auroral spectra by the OSIRIS spectrograph on the Odin spacecraft, offering another opportunity to perform the ratio measurement. Because of the considerable wavelength separation between these two atomic lines, precise instrumental relative response calibrations can be problematic. To maintain accurate on-orbit spectral calibrations, an atmospheric radiation model with multiple Rayleigh scatter is employed to constantly track instrumental response. An example of a calibrated single bright auroral spectrum over the full OSIRIS wavelength range of 275 to 815 nm, limb tangent altitude 105 km, is presented. Using a number of individual auroral spectra, the observed OI 557.7 nm brightness is plotted versus the observed OI 297.2 nm brightness to both verify the required linear relationship and to experimentally determine the brightness ratio. Spectral contamination by other auroral emission features is removed. From the linear fit, the observed B(557.7)/B(297.2) ratio is 9.3 ± 0.5. By comparison, a ratio of 9.8 ± 1 was recently reported, determined by combining results from a number of observational databases separated in time and in wavelength coverage.