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We summarize observations of around a thousand solar energetic particle (SEP)events since 1967 that include ~25 MeV protons, made by various near-Earthspacecraft (IMPs 4, 5, 7, 8, ISEE 3, SOHO), that encompass Solar Cycle 20 tothe current cycle (24). We also discuss recent observations of similar SEPevents in Cycle 24 made by the STEREO spacecraft. The observations show, forexample, that the time distribution of ~25 MeV proton events varies from cycleto cycle. In particular, the time evolution of the SEP occurrence rate in Cycle24 is strongly asymmetric between the northern and southern solar hemispheres,and tracks the sunspot number in each hemisphere, whereas Cycle 23 was moresymmetric. There was also an absence of 25 MeV proton events during the solarminimum preceding Cycle 24 (other minima show occasional, often reasonablyintense events). So far, events comparable to the exceptionally intense eventsdetected in Cycles 22 and 23 have not been observed at Earth in Cycle 24,though Cycle 21 (the largest of the cycles considered here) also apparentlylacked such events. We note a correlation between the rates of intense 25 MeVproton events and "ground level enhancements" (GLEs) observed by neutronmonitors, since 1967, and conclude that the number of "official" GLEs observedto date in Cycle 24 appears to be significantly lower than expected (5 to7+/-1) based on the rate of intense 25 MeV proton events in this cycle.

25 MeV Solar Proton Events in Cycle 24 and Previous Cycles