Odd hydrogen response thresholds for indication of solar proton and electron impact in the mesosphere and stratosphere

Abstract. Understanding the atmospheric forcing from energetic particle precipitation (EPP) is important for climate simulations on decadal time scales. However, presently there are large uncertainties in energy-flux measurements of electron precipitation. One approach to narrow these uncertainties is by analyses of EPP direct atmospheric impacts and their relation to measured EPP fluxes. Here we use odd hydrogen observations from the Microwave Limb Sounder and Whole Atmosphere Community Climate Model simulations, together with EPP fluxes from the GOES and POES satellites, to determine the response thresholds to solar proton events (SPEs) and radiation belt electron (RBE) precipitation. We consider a range of altitudes in the middle atmosphere, and all magnetic latitudes from pole to pole. We find that the lower flux limits for day-to-day EPP impact detection using OH and HO₂ are of the order of 10² protons/cm²/s/sr (E > 10 MeV) and 10⁴ electrons/cm²/s/sr (E = 100–300 keV). Based on the simulations, nighttime OH and HO₂ are good EPP indicators in the polar regions, and provide best coverage in altitude and latitude. Due to larger background concentrations, daytime detection requires larger EPP fluxes and is possible in the mesosphere only. SPE detection is easier than RBE detection because a wider range of polar latitudes is affected. We also find that MLS OH observations indicate a clear nighttime response to SPE and RBE in the mesosphere, similar to the simulations, while HO₂ data are overall too noisy for confident EPP detection.