The Role of Successive and Interacting CMEs in the Acceleration and Release of Solar Energetic Particles: Multi-Viewpoint Observations

Gradual and large solar energetic particle (SEP) events (with flux of ions with energy > 10 MeV above 10 pfu) are primarily produced in shocks driven by fast and wide coronal mass ejections (CMEs). Past researches, both in theory and statistics, have found that the situation where a primary, fast CME (priCME) is preceded by previous CMEs (preCMEs) is favorable to a more efficient particle acceleration. However, the physical causes of this association is still a matter of debate, including the association of the acceleration and the release of SEPs with the interaction of successive CMEs. Taking advantage of the twin Solar TErrestrial RElations Observatory spacecraft, we study 41 large SEP events in solar cycle 24 by multi-viewpoint observations. Although 21 events (~51%) have a preCME identifiable in the Large Angle and Spectrometric Coronagraph, we determine that the priCMEs overlap the preCMEs in three-dimension (3D) for only eleven events (~27%). We further investigate the acceleration (using type II radio bursts) and release (using velocity dispersion analysis) of the particles for all potential instances of CME-CME interaction in 3D. We find that, for six of eleven events, the priCME is far away from catching up with the preCME when the particles are released. However, for the limited samples, the SEP peak intensity is significantly higher in the events in which the priCME is closest to impact the preCME. We propose that the efficient acceleration and the following release of the SEPs do not require a meaningful interaction between CMEs, but the increased seed population or more enhanced turbulence levels occurring closer to the preCME can explain our findings.