25 MeV Solar Proton Events in Cycle 24 and Previous Cycles

TL;DR

This paper reviews over five decades of solar energetic particle events, focusing on 25 MeV protons, highlighting cycle-to-cycle variations, hemispheric asymmetries, and the lower-than-expected occurrence of ground level enhancements in Cycle 24.

Contribution

It provides a comprehensive analysis of SEP event patterns across multiple solar cycles, emphasizing hemispheric asymmetries and the discrepancy in GLEs during Cycle 24.

Findings

Cycle 24 shows hemispheric asymmetry in SEP occurrence.

No exceptionally intense events observed in Cycle 24 so far.

Ground level enhancements are fewer than expected in Cycle 24.

Abstract

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), 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 ~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). 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 observed to date in Cycle 24 appears to be significantly lower than expected (5 to 7+/-1) based on the rate of intense 25 MeV proton events in this cycle.