First evidence of middle atmospheric HO2 response to 27 day solar cycles from satellite observations

HO2 and OH, also known as HOx, play an important role in controlling middle atmospheric O-3. Due to their photochemical production and short chemical lifetimes, HOx are expected to respond rapidly to solar irradiance changes, resulting in O-3 variability. While OH solar cycle signals have been investigated, HO2 studies have been limited by the lack of reliable observations. Here we present the first evidence of HO2 variability during solar 27 day cycles by investigating the recently developed HO2 data from the Aura Microwave Limb Sounder (MLS). We focus on 2012-2015, when solar variability is strong near the peak of Solar Cycle 24. The features of HO2 variability, with the strongest signals at 0.01-0.068 hPa, correlate well with those of solar Lyman a. When continuous MLS OH observations are not available, the new HO2 data could be a promising alternative for investigating HOx variability and the corresponding impacts on O-3 and the climate.