Energetic Particle Precipitation (EPP) consists of highly energetic protons, electrons and neutrons that precipitate into the Earth's atmosphere from extra-terrestrial sources. Solar Proton Events (SPEs) is a type of EPP where a large quantity of protons are ejected from the Sun at high energies (10-300-MeV). The SPEs are sporadic but tend to occur more likely during solar maximum. In the near-Earth space, the energetic protons are guided by the Earth's magnetic field to the polar regions, typical energies providing access to 60 E   geomagnetic latitudes (Verronen et al., 2007). Depending on the energy of the protons, they can penetrate into the upper and middle atmosphere, and have a significant direct impact at altitudes above 30 km (Jia et al., 2020). The energetic protons cause ionization and dissociation in the middle atmosphere that ultimately lead to ozone reduction (Sinnhuber et al., 2012).The SPE impact on middle atmospheric odd oxygen,+ O) has been studied since the November 1969 SPE (Weeks et al., 1972), and is generally well known (Funke et al., 2011;Jackman et al., 2001;Seppälä et al., 2004). The SPE-driven O x Edepletion is mainly caused by the increased production of odd hydrogen,

<p>Energetic particle precipitation (EPP) impact on the middle atmospheric ozone chemistry plays potentially an important role in the connection between space weather and Earth's climate system. A variant of the Whole Atmosphere Community Climate Model (WACCM-D) implements a detailed set of ionospheric D-region chemistry instead of a simple parameterization used in the earlier WACCM versions, allowing to capture the impact of EPP in more detail, thus improving the model for long-term climate studies. Here, we verify experimentally the ion chemistry of the WACCM-D by analysing the middle atmospheric ozone response to the EPP forcing during well-known solar proton events<span> </span>(SPEs). We use a multi-satellite approach to derive the middle atmospheric sensitivity for the SPE forcing as a statistical relation between the solar proton flux and the consequent ozone change. An identical sensitivity analysis is carried out for the WACCM-D model results, enabling one-to-one comparison with the results derived from the satellite observations. Our results show a good agreement in the sensitivity between satellites and the WACCM-D for nighttime conditions. For daytime conditions, we find a good agreement for the satellite data sets that include the largest SPEs (max proton flux >10^<span>4 </span> pfu). However, for those satellite data-sets with only minor and moderate SPEs, WACCM-D tends to underestimate the sensitivity in daytime conditions. In summary, the comparisons WACCM-D ion chemistry, combined with the transportation, demonstrates a realistic representation of the SPE sensitivity of ozone, and thus provides a conservative platform for long-term EPP impact studies.</p>

A Solar Proton Event (SPE) occurs when, following a solar flare and coronal mass ejection, large quantities of protons are emitted toward the Earth and then subsequently accelerated to high energies (10-300 MeV). In near-Earth space, the protons are guided by the Earth's magnetic field to the polar regions. Their typical energies allow them to enter >60° geomagnetic latitudes (Verronen et al., 2007). Depending on their energies, they penetrate the upper and middle atmosphere and cause a direct significant impact at altitudes above 30 km (Jia et al., 2020). There, they cause ionization and dissociation in the middle atmosphere which leads to O 3 depletion (Sinnhuber et al., 2012).Since 1969, the SPE impact on middle atmospheric odd oxygen, O x (O 3 + O) has been studied (Weeks et al., 1972), and is overall a well-known phenomena (Funke et al., 2011;Jackman et al., 2001;Seppälä et al., 2004). In general, SPE causes increased production of odd hydrogen, HO x (H + OH + HO 2 ), and odd nitrogen, NO x (N + NO + NO 2 ), which then depletes O x through catalytic reactions. The increased HO x production happens through ion chemistry with molecular oxygen and water clusters reactions (