Solar particle events and their radiation threats

Energetic particles from the Sun have only been studied in detail during the last three decades. The modern record is good, although the number of the largest solar particle events are very few. The nuclides made by solar energetic particles in lunar rocks have been used to extend the record of these particles back /spl sim/10/sup 7/ years. The modern and ancient records are similar. By combining both sets of data, it has been inferred that solar particle events much larger than the largest events observed during the last four solar cycles are very rare.