First simultaneous retrieval of O₂ A-band nightglow volume emission rate and temperature in the MLT from Tianlu-1/HDI hyperspectral observations

Abstract. The O₂ A-band nightglow contains complementary information on volume emission rate (VER) and temperature in the mesosphere and lower thermosphere (MLT), yet their simultaneous retrieval remains challenging because it requires detecting weak airglow emissions while resolving temperature-sensitive rotational structures. The Hyperspectral Atmospheric Density Imager (HDI), launched aboard Tianlu-1 in January 2025, is China’s first spaceborne limb-sounding payload based on spatial heterodyne spectroscopy and provides hyperspectral limb observations of the O₂ A-band nightglow at a spectral resolution as fine as 0.0394 nm, enabling detailed characterization of its rotational structure. Here, we present a joint algorithm for simultaneous nighttime VER and temperature retrieval and demonstrate its performance using early on-orbit HDI measurements. Results show stable simultaneous retrievals during selected nighttime windows over 86–98 km. Retrieved VER distributions are consistent with the expected primary O₂ A-band emission layer. HDI temperatures agree with collocated, window-averaged SABER observations, with monthly mean absolute differences below 3.82 K. Monthly mean posterior 1σ uncertainties are below 5.09 % for VER and 3.51 K for temperature, with monthly mean temperature degrees of freedom for signal exceeding 0.946. These results demonstrate, for the first time, the simultaneous retrieval of nighttime VER and temperature from individual hyperspectral measurements, providing intrinsically co-registered and internally consistent constraints for subsequent photochemical retrievals of atomic oxygen. This advance establishes a new spaceborne capability for characterizing the coupled thermal–photochemical state of the MLT and investigating interactions among atmospheric chemistry, radiation, and dynamics in this region.