Occurrence probability density function (OPDF) of solar energetic particles with the annual F(>30 MeV) fluence exceeding the giving value. The triangles denote OPDF based on the data for the space era (Usoskin & Kovaltsov 2012). The circles correspond to the SEP events estimated from the terrestrial cosmogenic isotope data (modified after Usoskin 2017). Open symbols indicate the measured/estimated values, filled symbols indicate a conservative upper bound. Error bars bound the 68% confidence interval. The red hatched area encompasses the OPDF estimated in this work from ²⁶Al in lunar samples.