Occurrence of extreme solar particle events: Assessment from historical proxy data

Abstract

The probability of occurrence of extreme solar particle events (SPEs) with proton fluence (>30 MeV) F30 ≥ 1010 cm-2 is evaluated based on data on the cosmogenic isotopes 14C and 10Be in terrestrial archives covering centennial-millennial timescales. Four potential candidates with F30 = (1-1.5) × 1010 cm-2 and no events with F30 > 2 × 1010 cm-2 are identified since 1400 AD in the annually resolved 10Be data. A strong SPE related to the Carrington flare of 1859 AD is not supported by the data. For the last 11,400years, 19 SPE candidates with F 30 = (1-3) × 1010 cm-2 are found and clearly no event with F30 > 5 × 10 10 cm-2 (50 times the SPE of 1956 February 23) has occurred. These values serve as observational upper limits on the strength of SPEs on the timescale of tens of millennia. Two events, ca. 780 and 1460 AD, appear in different data series making them strong candidates for extreme SPEs. We build a distribution of the occurrence probability of extreme SPEs, providing a new strict observational constraint. Practical limits can be set as F 30 ≈ 1, 2-3, and 5×1010 cm-2 for occurrence probabilities ≈ 10-2, 10-3, and 10 -4 yr-1, respectively. Because of the uncertainties, our results should be interpreted as a conservative upper limit on the SPE occurrence near Earth. The mean solar energetic particle (SEP) flux is evaluated as ≈40 (cm2 s)-1, in agreement with estimates from lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence. © 2012 The American Astronomical Society. All rights reserved.