Gnevyshev gap effects in solar energetic particle activity

Time dependence of many solar activity factors and related phenomena has a double-peak structure around the maximum phase of the 11-year cycle. A distinctive minimum between peaks was named the Gnevyshev gap. This paper deals with Gnevyshev gap effects in solar energetic particle activity during the last four solar cycles. A hypothesis that the Gnevyshev gap structure is a result of superposition of quasi-biennial and 11-year oscillation is examined. The occurrence rate of solar energetic particle events does not contradict the hypothesis given all the events with the peak proton intensity J(> 10 MeV) > 1 cm(-2) s(-1) sr(-1) are considered. However, the most powerful solar energetic particle events (ground level enhancements) demonstrate pronounced peak and valley structure which is sometimes not synchronous with quasi-biennial oscillations in other solar indices. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.