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The origin of two large peaks in the atmospheric radiocarbon ( 14 C) concentration at  AD  774/5 and 993/4 is still debated. There is consensus, however, that these features can only be explained by an increase in the atmospheric  14 C production rate due to an extraterrestrial event. Here we provide evidence that these peaks were most likely produced by extreme solar events, based on several new annually resolved  10 Be measurements from both Arctic and Antarctic ice cores. Using ice core  36 Cl data in pair with  10 Be, we further show that these solar events were characterized by a very hard energy spectrum with high fluxes of solar protons with energy above 100 MeV. These results imply that the larger of the two events ( AD  774/5) was at least five times stronger than any instrumentally recorded solar event. Our findings highlight the importance of studying the possibility of severe solar energetic particle events.

Multiradionuclide evidence for the solar origin of the cosmic-ray events of AD 774/5 and 993/4