Ahmedabad 380009

Abstract. A statistical analysis of the contemporary (1954–1975) solar flare particle events has been made for the parameters F (integrated, proton fluence in cm–2 in an event with kinetic energy above 10 MeV) and R0 (the characteristic rigidity). These data are compared with the long-term averaged values determined from stable- and radio-nuclide measure-ments of lunar samples. The analysis shows that the ancient solar flare proton spectrum was harder (higher R0 values) compared to that observed in contemporary flares. A similar analysis can not be made for the mean long-term averaged flux (J, cm–2 S–1), since the contemporary averages suffer from an uncertainty due to the statistics of a single event. However, the average flux estimates for time durations 〈T 〉 exceeding 103 yr, are free from such uncertainties. The long-term averaged J values obtained over different time scales (104 – 106 yr) suggest a possible periodic variation in solar flare activity, with enhanced flux level during the last 105 yr. The available data rule out the occurrence of giant flares, with proton fluence exceeding 1015 cm–2 during the last million years. Key words: solar flares—solar activity—long term variation