HO ₂ Generation Above Sprite-Producing Thunderstorms Derived from Low-Noise SMILES Observation Spectra

No direct observational evidence of sprite-produced active radicals has been presented owing to the difficulty of observing a small event area in the nighttime mesosphere, whereas sprite chemical models have indicated that sprite discharge locally affects the atmospheric composition. We present the first observational evidence of a HO₂ production above sprite-producing thunderstorms from the coincidence of temporal-spatial observations of HO₂ spectra, sprite events, and thunderstorms by two space instruments, a submillimeter-wave limb spectrometer and ultraviolet/visible Imager and a ground-based very low frequency radiation lightning detection network. A total of three areas was identified with enhanced HO₂ levels of approximately 10²⁵ molecules. A chemical sprite model indicates an increase in HO₂ in the considered altitude region; however, the predicted production due to a single sprite event is smaller than the observed enhancement. Our observational results suggest that sprites potentially contribute 1% of nighttime background HO₂ generation at altitudes of 75–80 km globally.