Assessment of the radiation risk at flight altitudes for an extreme solar particle storm of 774 AD

Intense solar activity can lead to an acceleration of solar energetic particles and accordingly in crease in the complex radiation field at commercial aviation flight altitudes. We considered here the strongest ever observed event, namely that of 774 AD registered on the basis of cosmogenic isotope measurements, and computed the ambient dose at aviation altitude(s). Since the spectrum of solar protons during 774 AD event can not be directly obtained, as a first step, we derived the spectra of the solar protons during the GLE #5, the strongest event observed by direct measurements, which was subsequently scaled to the size of the 774 AD event and eventually used as input to the corresponding radiation model. The GLE #5 was considered as a conservative approach because it revealed the hardest-ever derived energy spectrum. The global map of the ambient dose was computed under realistic data-based reconstruction of the geomagnetic field during the 774 AD epoch, based on paleomagnetic measurements. A realistic approach on the basis of a GLE #45 was also considered, that is by scaling an event with softer spectra and lower particle fluxes compared to the GLE#5. The altitude dependence of the event integrated dose at altitudes from 30 kft to 50 kft was also computed for the both scenarios. The presented here study of the radiation effects during the extreme event of 774 AD give the necessary basis to be used as a reference to assess the worst-case scenario for a specific threat, that is radiation dose at flight altitudes.