First radiative transfer analysis of the OI 130.4-nm emission observed by NASA's TIMED mission

The OI 130.4-nm emission is one of the most prominent features in the far ultraviolet spectrum of terrestrial planets such as Earth, Mars, and Venus. Since the early era of space exploration in the 1960s, it has been recognized that this bright airglow emission could be used as a valuable diagnostic of the atomic oxygen abundance in planetary atmospheres. Atomic oxygen is the dominant neutral species in the upper atmosphere of terrestrial planets, whose distribution has critical impacts on the thermal structure, composition, chemistry, and dynamics of the planets' atmosphere and ionosphere as well as on spacecraft orbital dynamics and aerobraking maneuvers. However, the very large optical depth on the order of ~10⁴ - 10⁵ at the center of the OI 130.4-nm resonance lines had presented a formidable challenge to the interpretation of their observations. Particularly, over the last two decades NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission has yielded a comprehensive set of measurements of terrestrial OI 130.4-nm emission with unprecedented spatial and temporal coverage that has yet to be analyzed. In this study, we develop a comprehensive Monte Carlo Radiative Transfer (MCRT) model based on the algorithm of Meier and Lee [Planet. Space Sci., 30, 439-450, 1982]. The MCRT model is first validated against existing models and data, and is then applied to analyze for the first time the OI 130.4-nm emissions observed by the Global UltraViolet Imager (GUVI) aboard NASA's TIMED mission. The solar 130.4-nm flux that is required for the analysis is obtained from the Solar EUV Experiment (SEE) on the same mission. Our analysis reveals significant problems in the current understanding of the production of the OI 130.4-nm dayglow emission. Data-model discrepancies also suggest that under certain conditions the NRLMSISE-00 model may underestimate the thermospheric O densities, and/or that the GUVI and SEE calibrations may need to be revisited. We will also discuss the research efforts that are needed to reconcile models and observations in order to enable accurate remote sensing of the atomic oxygen abundance using the OI 130.4-nm emission in future space missions.