A new data set of nighttime chemical heating rates in the upper mesosphere and lower thermosphere derived from SCIAMACHY OH (9–6) emissions and SABER profiles

Abstract. Chemical heating from exothermic reactions is a key component of the upper mesosphere–lower thermosphere (UMLT) energy budget, yet its quantification remains uncertain. We derive a new data set of heating rates at 22:00 local time for seven major reactions using Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) OH(9–6) limb emissions, collocated with Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature and ozone profiles. The retrieval assumes chemical equilibrium for ozone and HO_x and applies updated Einstein coefficients from HITRAN-2020. Consistent with earlier studies, the relative importance of individual reactions varies systematically with altitude: hydrogen + ozone reaction (H + O₃) provides the leading contribution below ~ 92 km, whereas three-body oxygen recombination (O + O + M) dominates above this level. Other reactions make a substantial contribution across much of the 80–96 km region, accounting for roughly one-third to one-half of the total heating above ~ 86 km. The derived latitude-altitude heating structures of the dominant reactions are significantly modulated by atmospheric tides. In the equatorial zone, these heating rates exhibit a pronounced semiannual cycle that tracks seasonal changes in temperature and key reactants. Relative to previous SCIAMACHY-based estimates, the updated data set yields lower heating rates from H + O₃. An uncertainty assessment indicates ~ 30 % uncertainty for H + O₃ and ~ 45–60 % for O + O + M. These results refine and consolidate current understanding of chemical heating and its variability in the UMLT.