Interpreting Observed and Modeled Local Time Variations in Thermospheric Molecular Oxygen

Abstract This paper investigates the strong diurnal signature in thermospheric molecular oxygen () density observed by the Global‐scale Observations of Limb and Disk (GOLD) mission. Comparing GOLD local time variations against three independent thermospheric data sets between 130 and 200 km at low and middle latitudes using NRLMSIS as a mediator, we found that the GOLD diurnal variation is inconsistent with these data sets and NRLMSIS. Further comparisons with the National Center for Atmospheric Research (NCAR) thermosphere general circulation models (TGCMs), constrained by either reanalysis or recent satellite observations, show simulated exhibits a semidiurnal structure, similar but out‐of‐phase with NRLMSIS, and disagreeing with GOLD. Force term analysis of the continuity in one of the NCAR TGCMs revealed that local time variations are predominantly driven by vertical advection, offering the best explanation for what could be causing the diurnal signature observed by GOLD. While, this paper does not unequivocally determine the cause for the discrepant GOLD local time behavior, we found that magnetic field, instrument temperature, and retrieval algorithm effects do not introduce the observed local time bias in the GOLD's measurements. Given the discrepancy in the local time structure from GOLD compared to all other thermospheric data sources, further investigation is warranted to determine whether it stems from a gap in our understanding of thermospheric behavior or an unresolved issue with the GOLD data.