Investigating the Causes of Solar-Cycle Variations in Solar Energetic Particle Fluences and Composition

Measurements with the ACE, STEREO, and GOES spacecraft during the first 5.8 years of solar cycle 24 show that the number of large Solar Energetic Particle (SEP) events is reduced by ~32% compared to this point of cycle 23, while the fluences of >10 MeV/nucleon ions from H to Ni are reduced by factors ranging from 4 to ~10. A comparison of H, O, and Fe energy spectra from the ten largest events of the two cycles shows that the spectral breaks that are typically observed in SEP energy spectra are occurring ~3 times lower in energy/nucleon than in cycle 23. We investigate the origin of these cycle-to-cycle fluence, spectral and composition differences by evaluating possible factors that include: 1) the properties of the associated CMEs; 2) the interplanetary magnetic field strength; and 3) the density of suprathermal seed particles. These properties are evaluated in the context of existing SEP acceleration models. We conclude that both the reduced magnetic field strength and the reduced seed particle densities are contributing to the reduction in SEP output during cycle 24. In particular, we point out that in the standard model for SEP shock acceleration the maximum energy achieved is a strong function of the rate at which protons are injected into the shock acceleration process.