Gnevyshev-Ohl rule for strong solar proton events

Data on strong solar proton events, including: (a) instrumental record of ground level enhancements (1954-1996), (b) proxy record derived from nitrate series (1700-1944) and (c) proxy record derived from 10Be series (1755-1927) were analyzed. It was shown that strong solar proton events are more frequent during even-numbered solar cycles than during odd-numbered ones with a probability of more than 0.93. The results provide new evidence for the fundamental character of the Gnevyshev-Ohl rule. Possible causes of the revealed even-odd effect are discussed.