http://arxiv.org/abs/1604.07873
We summarize observations of around a thousand solar energetic particle (SEP) events since 1967 that include ~25 MeV protons made by various near-Earth spacecraft (IMPs 4, 5, 7, 8, ISEE 3, SOHO), encompassing solar cycles 20 to the current cycle (24). We also discuss recent observations of similar SEP events in cycle 24 from the STEREO spacecraft. The observations show, for example, that the time distribution of 25 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 25 MeV proton events during the solar minimum preceding cycle 24 (other minima show occasional, often reasonably intense events), and, so far, there have been few exceptionally intense events in cycle 24 compared to cycles 22 and 23, though cycle 21 also apparently lacked such events. We note a correlation between the rate of intense 25 MeV proton events and “ground level events” observed by neutron monitors since 1967 and demonstrate that the GLE rate at Earth in cycle 24 to date appears to be significantly lower than expected based on the intense 25 MeV proton event rate.
I. Richardson, T. Rosenvinge and H. Cane
Thu, 28 Apr 16
3/57
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