Abstract
The long-term variation of the solar activity and its nature are statistically analysed by using the data on the relative sunspot numbers for the last two hundred years. Based on this analysis, the occurrence frequency of large solar flares in the past is estimated to find a clue to deduce the production rate of high-energy particles from the Sun in the past. According to the results from the present study, during the period that the solar activity was much higher than that observed during solar cycle No. 19, this production rate must have been very much higher than that being currently observed. Such a period might have been hazardous to life on Earth because of the destruction of the ozone layer by the bombardment of solar cosmic rays.
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