Discovery of an orange FeO feature in the night airglow spectrum with the OSIRIS spectrograph on Odin
Since the instrument is a CCD spectrograph, all wavelengths are exposed simultaneously thus avoiding the effect of temporal intensity variations inherently present in spectrally scanning instruments. The relative spectral sensitivity over the entire wavelength range has been quantified to yield an estimated 5% precision. In order to maintain accurate on-orbit spectral calibrations an atmospheric radiation model with multiple Rayleigh scattering is employed to regularly update the OSIRIS spectral response throughout the mission. Averages of spectra at a series of tangent limb altitudes were assembled from numerous limb scans at low latitudes. Limb radiance altitude profiles for a number of observed spectral features were obtained from these averaged spectra. These radiance profiles were inverted to obtain volume emission rate altitude profiles. Synthetic spectra for the hydroxyl and O2 Herzberg airglow emission bands were generated and scaled with the observed band intensities to remove these known airglow components in order to isolate the underlying airglow feature. Tropical latitudes were chosen to minimize the classic green airglow continuum from the reaction of nitric oxide with atomic oxygen since nitric oxide is small at 90 km in the tropics.
Other potential photochemical sources of the orange glow are reviewed and their likely importance assessed as a mechanism for the current observations. A comparison of the observed spectrum feature with laboratory measurements of the FeO emission spectrum from 530 to 650 nm shows a remarkable similarity, positively identifying FeO from iron and ozone as the source of the orange airglow feature. The same orange feature has been observed in the afterglow of meteorite trains. The similarity of of the meteorite afterglow spectrum with the new airglow feature was demonstrated by comparison of the two.
The measured volume emission rate altitude profiles of the FeO feature were compared with altitude profiles of the atomic sodium emission at 590 nm and OH emission bands in the same OSIRIS spectrum; all three of these profiles are similar. This comparison is consistent with the laboratory excitation mechanism of atomic iron with ozone since all three emissions originate from the reaction of atoms with ozone.
Examination of several airglow spectra measured by other investigators showed confirmatory evidence of the presence of the orange FeO feature even though it has not been previously identified as such.