Articles | Volume 17, issue 6
Research article
28 Mar 2017
Research article |  | 28 Mar 2017

Measuring FeO variation using astronomical spectroscopic observations

Stefanie Unterguggenberger, Stefan Noll, Wuhu Feng, John M. C. Plane, Wolfgang Kausch, Stefan Kimeswenger, Amy Jones, and Sabine Moehler

Abstract. Airglow emission lines of OH, O2, O and Na are commonly used to probe the MLT (mesosphere–lower thermosphere) region of the atmosphere. Furthermore, molecules like electronically excited NO, NiO and FeO emit a (pseudo-) continuum. These continua are harder to investigate than atomic emission lines. So far, limb-sounding from space and a small number of ground-based low-to-medium resolution spectra have been used to measure FeO emission in the MLT. In this study the medium-to-high resolution echelle spectrograph X-shooter at the Very Large Telescope (VLT) in the Chilean Atacama Desert (24°37′ S, 70°24′ W; 2635 m) is used to study the FeO pseudo-continuum in the range from 0.5 to 0.72 µm based on 3662 spectra. Variations of the FeO spectrum itself, as well as the diurnal and seasonal behaviour of the FeO and Na emission intensities, are reported. These airglow emissions are linked by their common origin, meteoric ablation, and they share O3 as a common reactant. Major differences are found in the main emission peak of the FeO airglow spectrum between 0.58 and 0.61 µm, compared with a theoretical spectrum. The FeO and Na airglow intensities exhibit a similar nocturnal variation and a semi-annual seasonal variation with equinoctial maxima. This is satisfactorily reproduced by a whole atmosphere chemistry climate model, if the quantum yields for the reactions of Fe and Na with O3 are 13 ± 3 and 11 ± 2 % respectively. However, a comparison between the modelled O3 in the upper mesosphere and measurements of O3 made with the SABER satellite instrument suggests that these quantum yields may be a factor of ∼ 2 smaller.

Short summary
This study focuses on the analysis of astronomical medium-resolution spectra from the VLT in Chile to measure airglow pseudo-continuum emission of FeO in the optical regime. Compared to OH or Na emissions, this emission is difficult to measure. Using 3.5 years of spectroscopic data, we found annual and semi-annual variations of the FeO emission. Furthermore, we used WACCM to determine the quantum yield of the FeO-producing Fe + O3 reaction in the atmosphere, which has not been done before.
Final-revised paper