OCCURRENCE OF EXTREME SOLAR PARTICLE EVENTS: ASSESSMENT FROM HISTORICAL PROXY DATA

The probability of occurrence of extreme solar particle events (SPEs) with proton fluence (>30 MeV) F-30 >= 10(10) cm(-2) is evaluated based on data on the cosmogenic isotopes C-14 and Be-10 in terrestrial archives covering centennial-millennial timescales. Four potential candidates with F-30 = (1-1.5) x 10(10) cm(-2) and no events with F-30 > 2 x 10(10) cm(-2) are identified since 1400 AD in the annually resolved Be-10 data. A strong SPE related to the Carrington flare of 1859 AD is not supported by the data. For the last 11,400 years, 19 SPE candidates with F-30 = (1-3) x 10(10) cm(-2) are found and clearly no event with F-30 > 5 x 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 approximate to 1, 2-3, and 5x10(10) cm(-2) for occurrence probabilities approximate to 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 approximate to 40 (cm(2) s)(-1), in agreement with estimates from lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence.