Abstract
<p>Atmospheric effects of solar proton events (SPEs) have been studied for decades, because their drastic impact can be used to test our understanding of upper stratospheric and mesospheric chemistry in the polar cap regions. For example, odd hydrogen and odd nitrogen are produced during SPEs, which leads to depletion of ozone in catalytic reactions, such that the effects are easily observed from satellites during the strongest events. Until recently, the complexity of the ion chemistry in the lower ionosphere (i.e., in the D region) has restricted global models to simplified parameterizations of chemical impacts induced by energetic particle precipitation (EPP). Because of this restriction, global models have been unable to correctly reproduce some important effects, such as the increase in mesospheric HNO<sub>3</sub> or the changes in chlorine species. Here we use simulations from the WACCM-D model, a variant of the Whole Atmosphere Community Climate Model, to study the statistical response of the atmosphere to the 66 strongest SPEs which occurred in the years 1989–2012. Our model includes a set of D-region ion chemistry, designed for a detailed representation of the atmospheric effects of SPEs and EPP in general. We use superposed epoch analysis to study changes in O<sub>3</sub>, HO<sub>x</sub> (OH + HO<sub>2</sub>), Cl<sub>x</sub> (Cl + ClO), HNO<sub>3</sub>, NO<sub>x</sub> (NO + NO<sub>2</sub>) and H<sub>2</sub>O. Compared to the standard WACCM which uses an ion chemistry parameterization, WACCM-D produces a larger response in O<sub>3</sub> and NO<sub>x</sub> and a weaker response in HO<sub>x</sub> and introduces changes in HNO<sub>3</sub> and Cl<sub>x</sub>. These differences between WACCM and WACCM-D highlight the importance of including ion chemistry reactions in models used to study EPP. </p>
Highlights
Solar proton events (SPEs) are observed on Earth when highenergy protons accelerated in the sun’s magnetic field during a solar coronal mass ejection strike Earth
As the SPE effects are largely known from previous work, we focus on the improvement provided by additional ion chemistry reactions included in the Whole Atmosphere Community Climate Model (WACCM)-D chemistry
We present an analysis of the chemical impacts of SPE on the middle atmosphere using simulations from WACCM
Summary
Solar proton events (SPEs) are observed on Earth when highenergy protons accelerated in the sun’s magnetic field during a solar coronal mass ejection strike Earth. Due to the large number of D-region ions and ionic reactions, atmospheric models have typically included the SPE, or EPP, effects using simple parameterizations of HOx and NOx production. WACCM-D has been used to study mesospheric nitric acid and cluster ion composition during electron precipitation events (Orsolini et al, 2018) and magnetic latitude dependency of SPE ionospheric impact (Heino et al, 2019). While several of the largest SPEs included in the analysis have previously been studied individually, the statistical approach used here allows for inclusion of a number of moderate-sized events under various background atmosphere and illumination conditions. This approach allows for the identification of climatological effects above natural variability. As the analysis includes SPEs of different sizes occurring during different seasons, a statistical approach is most useful for the study of temporal and spatial extent, rather than magnitude of the response
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