Solar Energetic-Particle Ground-Level Enhancements and the Solar Cycle

Severe geomagnetic storms appear to be ordered by the solar cycle in a number of ways. They occur more frequently close maximum and declining phase, are common larger cycles show different patterns occurrence odd- even-numbered cycles. Our knowledge most extreme space weather events, however, comes from spikes cosmogenic-isotope ($^{14}$C, $^{10}$Be $^{36}$Cl) records that attributed significantly energetic particle (SEP) events than have been observed during age. Despite both SEPs being driven eruptive phenomena, event-by-event correspondence between is low. Thus it should not assumed priori found for also hold events. In this study we investigate trends timing magnitude 67 SEP ground-level enhancements (GLEs) recorded neutron monitors since mid 1950s. Using models GLE probability, GLEs around factor four likely minimum, they preferentially earlier odd-numbered There insufficient data conclusively determine whether produce GLEs. Implications putative space-weather discussed. We find tend cluster within few tens days, due particularly productive individual active regions, with approximately 11-year separations, owing ordering. But these timescales do explain which require multiple over consecutive years.