The impact of extreme solar events on the atmosphere

Besides the well-known 11 year solar cycle, the Sun occasionally produces strong eruptions on the Sun‘s surface and in the corona. They can first be seen as flare events in electromagnetic spectrum down to X ray wavelengths. Within these strong eruptions, particles in the solar plasma, mainly protons, are accelerated to high energies that hit the Earth within hours after the event. In addition, plasma clouds can be accelerated and ejected into the interplanetary space and, provided they are directed to the Earth, can cause severe geomagnetic disturbances. This results in a further energetic particle precipitation event a few days after the primary solar eruption. The strength of these events spans orders of magnitude, with the strongest having dramatic impact on the ionosphere and the middle atmosphere affecting even human activities. Here we study the chemical impact and dynamical of solar events on the middle atmosphere which are on the very extreme side but still within the range of a one per millennia event.

We first derive a reference example of an extreme solar event from historical records of solar proton events and from analyzed distributions of energy spectra for geomagnetic storms. We then take ionization rates calculated from strong observed events and scale them to represent the extreme events. Finally, we combine the solar proton event with the geomagnetic storm as both events typically impact different parts of the atmosphere. The ionization rates for the extreme event are then used in simulations in the KASIMA and EMAC model which both include energetic particle induced chemistry.In order to represent different dynamical situations in the middle atmosphere which are important for the vertical coupling between the mesosphere-lower thermosphere (MLT) region and the stratosphere we select specific periods of the ERA-Interim dataset with a special focus on sudden stratospheric warmings (SSW) and apply the event for those situations. The simplified production efficiency of NOx and HOx in the models is further compared to an ion chemistry model where the extreme ionization rates are applied. The case of a SSW which shows an elevated stratosphere synchronized with the extreme event is studied in detail as a kind of worst case scenario.