We summarize observations of around a thousand solar energetic particle ( SEP ) events since 1967 that include \sim 25  MeV protons , made by various near-Earth spacecraft ( IMPs 4 , 5 , 7 , 8 , ISEE 3 , SOHO ) , that encompass Solar Cycle 20 to the current cycle ( 24 ) .
We also discuss recent observations of similar SEP events in Cycle 24 made by the STEREO spacecraft .
The observations show , for example , that the time distribution of \sim 25 Â MeV proton events varies from cycle to cycle .
In particular , the time evolution of the SEP occurrence rate in Cycle 24 is strongly asymmetric between the northern and southern solar hemispheres , and tracks the sunspot number in each hemisphere , whereas Cycle 23 was more symmetric .
There was also an absence of 25Â MeV proton events during the solar minimum preceding Cycle 24 ( other minima show occasional , often reasonably intense events ) .
So far , events comparable to the exceptionally intense events detected 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 apparently lacked such events .
We note a correlation between the rates of intense 25 MeV proton events and “ ground level enhancements ” ( GLEs ) observed by neutron monitors , since 1967 , and conclude that the number of “ official ” GLEs ( 1 ) observed to date in Cycle 24 appears to be significantly lower than expected ( 5 to 7 \pm 1 ) based on the rate of intense 25 MeV proton events in this cycle .