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

Abstract

<jats:p>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 &gt; 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. </jats:p>

Sensitivity of Middle Atmospheric Ozone to Solar Proton Events: A Comparison Between a Climate Model and Satellites

“…As discussed in Section 2, MEE ionization is not included in our simulations, which could in principle explain some of the model-observation differences. However, we argue that including MEE in our simulations 10 of 12 would have minor, if any, impact on our results: First, the majority of the MEE direct short-term HO x E response impact on ozone are restricted within 55-65 E  magnetic latitudes (Häkkilä et al, 2020). In our analysis, we utilize ozone data sampled over the entire polar cap regions above 60 E  geographic latitudes.…”

“…In our analysis, we utilize ozone data sampled over the entire polar cap regions above 60 E  geographic latitudes. Second, the major MEE impact are seen at altitudes between 60 and 80 km altitude (Häkkilä et al, 2020), while in our analysis, we cover altitudes 35-90 km altitudes. Furthermore, considering that our main results (Figures 2 and 3) shows in general either an agreement throughout the altitude steps, or a disagreement that extends below 60 km altitude.…”

“…30 1000 E E   keV in our simulations. In any case, below 90 km altitude we expect that major SPEs will dominate over MEE in both magnitude as well as altitude and latitude extent of short-term atmospheric response (Häkkilä et al, 2020). We further discuss the omission of MEE and justify our approach in Section 5.…”

“…x E at altitudes from 40 to 90 km in particular (Häkkilä et al, 2020), with the peak of production typically being near 70 km altitude.…”