25 MeV solar proton events in Cycle 24 and previous cycles

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 (1) 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.

The Acceleration of High-energy Protons at Coronal Shocks: The Effect of Large-scale Streamer-like Magnetic Field Structures

“…While 16 GLE events were recorded in solar cycle 23, there have been only 2 GLEs observed so far in solar cycle 24 (Gopalswamy et al 2013a;Thakur et al 2014). Meanwhile, the number of large SEP events in this solar cycle is significantly smaller than that of the last solar cycle (e.g., Mewaldt et al 2015;Richardson et al 2017). It is known that the solar activity is weak in the current solar cycle, however, the physical reason for the lack of large SEP events and GLEs is still not clear (see, e.g., Gopalswamy et al 2013aGopalswamy et al , 2014Giacalone 2015;Mewaldt et al 2015).…”

Nowcasting of Solar Energetic Particle Events using near real-time Coronal Mass Ejection characteristics in the framework of the FORSPEF tool

“…Additionally, a larger fraction of halo CMEs is spotted among the CMEs of SC24 (Gopalswamy et al, 2014). At the same time, although CMEs are found to drive shocks, and that the number of SEP events with E > 10 and E > 100 MeV in SC 24 is comparable to the relevant number of previous cycles (Bazilevskaya et al, 2015;Richardson et al, 2016), those CMEs are not efficient in accelerating particles to higher energies (Paassilta et al, 2017). However, recently, Alberti et al (2017), showed that there is an observed 40% reduction in SEP activity between two selected intervals in SC23 (15.09.1996-15.09.2002) and SC24 (15.12.2008-15.12.2014.…”

Revising More Than 20 Years of EPHIN Ion Flux Data—A New Data Product for Space Weather Applications

“…Furthermore, it has to be noted that the vast majority of these major SEP events did occur in solar cycle 23 clearly underlining the unusual weak character of cycle 24 (Richardson et al, ). Similar effects have been also observed for the contribution to the fluence in previous studies (Mewaldt et al, , ).…”