Fluence Ordering of Solar Energetic Proton Events Using Cosmogenic Radionuclide Data
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While data on the cosmogenic isotopes 14C and 10Be made it possible to evaluate extreme solar proton events (SPEs) in the past, their relation to standard parameters quantifying the SPE strengths, viz. the integrated fluence of protons with energy above 30 MeV, F 30, is ambiguous and strongly depends on the assumed shape of the energy spectrum. Here we propose a new index, the integral fluence of an SPE above 200 MeV, F 200, which is related to the production of the cosmogenic isotopes 14C and 10Be in the Earth atmosphere, independently of the assumptions on the energy spectrum of the event. The F 200 fluence is reconstructed from past cosmogenic isotope data, which provides an assessment of the occurrence probability density function for extreme SPEs. In particular, we evaluate that extreme SPEs with F 200>1010 cm−2 occur no more frequently than once per 10 – 15 kyr.
KeywordsSolar energetic particles Cosmogenic radioisotopes
G.A. Kovaltsov is grateful to the Academy of Finland for partial support. I.G. Usoskin’s contribution was made in the framework of the ReSoLVE Centre of Excellence (Academy of Finland, project no. 272157). E.W. Cliver acknowledges support from the Air Force Office of Scientific Research. A.J. Tylka and W.F. Dietrich are supported by NASA Solar & Heliophysics SR&T under NNH12AT09I.
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