Comparing Energetic Storm Particle Heavy-ion Properties in Solar Cycles 23 and 24

We examine variations in energetic storm particle (ESP) heavy-ion average intensities and energy spectra between similar to 0.1 and 75 MeV nucleon(-1) at coronal mass ejection (CME)-driven interplanetary shocks for events observed at the ACE spacecraft. We compare ESP events observed during the weaker solar cycle (SC) 24 and the relatively stronger SC 23 to investigate any effects on the strength of an SC, including the associated transient events, on ESP properties at 1 au. We find that the number of clearly defined heavy-ion ESP events at ACE during SC 23 is about twice that observed during SC 24 (76 versus 41). The average transit speed of the driving interplanetary CMEs (ICMEs) at 1 au is 20% higher during SC 23 than during SC 24 (859.4 km s(-1) versus 729.1 km s(-1)). The correlation of ESP average intensities with ICME speeds shows that lower-speed ICMEs in SC 24 can be as efficient as the higher-speed events in SC 23 at producing ESPs below 2 MeV nucleon(-1). The distribution and magnitude of the average intensities for energies below similar to 1 MeV nucleon(-1) are consistent between both SCs. However, events with intensity enhancements at higher energies (>similar to 10 MeV nucleon(-1)) are more frequent and their intensity distributions are harder for SC 23, resulting in an increase in the rollover energy (E-0) for their spectra profiles. This suggests more efficient ESP acceleration at >10 MeV nucleon(-1) during SC 23.