An optimal estimation-based retrieval of upper atmospheric oxygen airglow and temperature from SCIAMACHY limb observations

Abstract

An optimal estimation-based algorithm is developed to retrieve the number density of excited oxygen (O₂) molecules that generate airglow emissions near 0.76 µm ($b^{\mathrm{1}}{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$ or A band) and 1.27 µm (a¹Δ_g or ¹Δ band) in the upper atmosphere. Both oxygen bands are important for the remote sensing of greenhouse gases. The algorithm is applied to the limb spectra observed by the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument in both the nominal (tangent heights below ∼ 90 km) and mesosphere–lower thermosphere (MLT) modes (tangent heights spanning 50–150 km). The number densities of emitting O₂ in the a¹Δ_g band are retrieved in an altitude range of 25–100 km near-daily in 2010, providing a climatology of O₂ a¹Δ_g-band airglow emission. This climatology will help disentangle the airglow from backscattered light in nadir remote sensing of the a¹Δ_g band. The global monthly distributions of the vertical column density of emitting O₂ in a¹Δ_g state show mainly latitudinal dependence without other discernible geographical patterns. Temperature profiles are retrieved simultaneously from the spectral shapes of the a¹Δ_g-band airglow emission in the nominal limb mode (valid altitude range of 40–100 km) and from both a¹Δ_g- and $b^{\mathrm{1}}{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$ -band airglow emissions in the MLT mode (valid range of 60–105 km). The temperature retrievals from both airglow bands are consistent internally and in agreement with independent observations from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), with the absolute mean bias near or below 5 K and root mean squared error (RMSE) near or below 10 K. The retrieved emitting O₂ number density and temperature provide a unique dataset for the remote sensing of greenhouse gases and constraining the chemical and physical processes in the upper atmosphere.