Solar coronal mass ejections can accelerate charged particles, mostly protons, to high energies, causing Solar Proton Events (SPEs). Such energetic particles can precipitate upon the Earth’s atmosphere, mostly in polar regions because of the geomagnetic shielding. Here, SPE induced chlorine activation due to ion-chemistry can occur and the activated chlorine depletes ozone in the polar middle atmosphere. These events can also influence the dynamics of the middle atmosphere across scales from variations in the EUV radiation to extreme solar particle events. We use state of the art 1D stacked-box Exoplanetary Terrestrial Ion Chemistry model (ExoTIC), of atmospheric ion and neutral composition to investigate such events in the Northern Hemisphere (NH). The Halloween SPE that occurred in late October 2003 is used as a test field for our study. This event has been extensively studied before using different 3D models and satellite observations. Our main purpose is to use such a large event that has been recorded by MIPAS on ENVISAT to evaluate the performance of the ion-chemistry model. Sensitivity tests were carried out for different model settings with a focus on the chlorine species of HOCl and ClONO2 as well as O3 and reactive nitrogen, NOy. The model simulations were performed in the Northern Hemisphere
at a high latitude of 67.5◦N, inside the polar cap. Comparison of the simulated effects against MIPAS observations for the Halloween SPE revealed a rather good temporal agreement, also in terms of altitude range for HOCl, O3 and NOy. For ClONO2, a good agreement was found in terms of altitude range. The model showed ClONO2 enhancements after the peak of the event. The best model setting was the one with full ion-chemistry where O(1D) was set to photo-chemical equilibrium. HOCl and ozone changes are very well reproduced by the model, specially for night-time. HOCl was found to be the main active chlorine species under night-time conditions resulting in an increase of more than 0.2 ppbv. Further, ClONO2 enhancements of 0.2-0.3 ppbv have been observed both during daytime and night-time. Model settings that compared best with MIPAS observations were applied to an extreme solar event that occurred in 775 A.D., presumably once in a 1000 year event and also to a high energetic electron precipitation (HEEP) event that occurred over the Moscow region. With the model applied to the extreme scenario, assessment can be made what is to be expected at worst for effects of a SPE on the middle atmosphere concentrating on effects of ion-chemistry compared to crude parameterisations. Here, a systematic analysis comparing the impact of the Halloween SPE and the extreme event on the Earth’s middle atmosphere is presented. As seen from the model simulations, both events were able to perturb the polar stratosphere and mesosphere, with a high production of NOy and HOx. Longer lasting and stronger stratospheric ozone loss was seen for the extreme event. Qualitative difference between the two events and a long lasting impact on HOCl and HCl for the extreme event was found. Chlorine ion-chemistry contributed to a stratospheric ozone loss of 2.4% for daytime and 10% for nighttime during the Halloween SPE as seen with time dependent ionisation rates applied to the model. Furthermore, while comparing the Halloween SPE and the extreme scenario, with ionisation rate profiles applied just for the event day, the inclusion of chlorine ion-chemistry added an ozone loss of 10% and 20% respectively. Studies were performed with the 3D chemistry climate model EMAC to assess the impact of particle forcings and EUV (extreme ultra violet solar flux) forcings during sudden stratospheric warmings (SSWs) to look at the response of coupling from the upper atmosphere. The EUV and thereby the photo-ionisation was varied, that dominates the changes during high solar activity. With EUV forcings, for the simulations with NOx based parameterisation, ozone levels decrease by 15% in the stratosphere with changing EUV. The results showed polar vortex strengthening after the event which was seen in the temperature increase and the strengthening of the zonal mean zonal wind in the stratosphere with changing EUV and that is due to dynamical feedback from ozone. The changes in the middle atmosphere were found to be quite significant.