A fast spectral line-ratio algorithm for mesospheric airglow rotational temperature retrieval and validation

Abstract. Airglow rotational temperatures are effective tracers of variability in the mesopause region. To enable efficient processing of long-duration, high-temporal-resolution observations from the Mesospheric Airglow Spectrum Photometer (MASP), a Fast Airglow Spectral Line-Ratio algorithm (FASLR) is developed for rotational temperature retrieval. FASLR uses forward-modeled synthetic spectra to establish ratio–temperature relationships, derives independent temperature estimates from multiple line-intensity ratios, and combines them with weights based on the relative sensitivity of each ratio to instrumental perturbations. By avoiding computationally intensive multi-parameter iterative optimization, the algorithm substantially improves computational efficiency and remains robust to occasional anomalies in individual spectral lines. FASLR is validated using two nights of co-located MASP and sodium fluorescence Doppler lidar observations, showing good consistency with the corresponding lidar temperatures. The algorithm is then applied to six nights of dual-channel MASP observations of the O₂ (0–1) and OH (6–2) emissions and compared with independent temperatures from SABER, Aura/MLS, SD-WACCMX, and NRLMSIS 2.1. Overall, these comparisons indicate general consistency in the background temperature level, the relative offset between the O₂ and OH emission layers, and the range of nocturnal variability. Near representative emission-layer altitudes, the O₂ rotational temperatures reproduce the nocturnal background evolution reasonably well, whereas the OH temperatures show a more evident early-night high bias but better agreement during the latter half of the night. Case studies further demonstrate that the FASLR-derived temperatures are suitable for quantitative analyses of both longer-period nocturnal variability and shorter-period disturbances. On 2 February 2024, temperature and intensity perturbations in the O₂ and OH layers exhibit a common dominant period band and stable phase relationships. On 7 February 2024, both layers show concurrent power enhancements within a similar period band, and simultaneous observations from the Mesospheric Airglow Wide-angle Imager (MAWI) independently support the identified disturbance. Overall, FASLR provides an efficient and robust approach for dual-channel airglow temperature retrieval and supports investigations of multi-scale wave processes in the mesopause region.