The Impact of a Solar Extreme Event on the Middle Atmosphere, a Case Study

Abstract. A possible impact of an extreme solar particle event (ESPE) on the middle atmosphere is studied for the present-day climate and geomagnetic conditions. We consider an ESPE with an occurrence probability of about 1 per millenium combined with an extreme geomagnetic storm (GMS) following the ESPE. The strongest known and best documented ESPE of 774/5 CE is taken as a reference example and established estimates of the corresponding ionization rates are applied. The ionization rates due to the energetic particle precipitation (EPP) during an extreme geomagnetic storm are up-scaled from analyzed distributions of electron energy spectra of observed geomagnetic storms. The consecutive buildup of NOx and HOx by the ionization is modeled in the high top 3D chemistry circulation model KASIMA, using specified dynamics from ERA-Interim analyses up to the stratopause. A specific dynamical situation was chosen which includes an elevated stratosphere event during January and maximizes the vertical coupling between the Northern polar mesosphere-lower thermosphere region and the stratosphere and therefore allows to estimate a maximum possible impact. The results show a strong enhancement of NOx which causes a substantial decrease of ozone in the mesosphere and stratosphere, and a significant decrease of total ozone in the Northern hemisphere in spring, enduring into the midlatitude summer after the event. The geomagnetic storm causes strong ozone reduction in the mesosphere but plays only a minor role for the reduction in total ozone. In the Southern hemisphere, the long-lived NOy in the polar stratosphere which is produced almost solely by the ESPE, is transported into the Antarctic polar vortex where it experiences strong denitrification into the troposphere. For this special case, we estimate a NO₃ wash-out which could produce a measurable signal in ice cores. The reduction in total ozone causes an increase of the UV erythema dose of less than 5 % which maximizes in spring for Northern latitudes of 30° and in summer for Northern latitudes of about 60°.