The Role of Coronal Mass Ejection Interactions in the Acceleration of Solar Energetic Particle Events

The role of multiple CME-driven shocks in the acceleration of solar energetic particle events has long been proposed to account for events such as the exceptionally intense August 1972 event. More recently, the availability of observations of coronal mass ejections from the LASCO coronagraphs has led to studies of the possible role of multiple CMEs in the acceleration of large SEP events. Typically these studies claim a strong association between the presence of a "preceding" CME prior to a wide, fast CME and the occurrence of a large SEP event e.g., one that appears on the NOAA list of events exceeding 10 pfu at >10 MeV. This has led to various hypotheses to "explain" this association such as acceleration occurring as a result of CME-CME interactions, the conditions in the wake of the preceding CME enhance the acceleration efficiency of the shock driven by the main CME, or the preceding CME provides a "seed" population for acceleration at this shock, aligning with current ideas that SEP events are accelerated by CME-driven shocks from a putative pre-existing seed population. However, a number of factors are not typically considered in such studies. For example: A wider sample of SEP events is necessary to determine whether the conditions necessary to generate the largest events (e.g., those on the NOAA list) differ from those that give rise to weaker events; It is possible that CMEs are typically preceded by other CMEs within the (typically ~1 day) interval often used in such studies, irrespective of whether they give rise to large SEP events; Active regions that give rise to large SEP events may be intrinsically more CME-productive, increasing the likelyhood that a preceding CME will be present; Is it reasonable to expect a relatively slow preceding CME to accelerate suprathermal seed particles that can strongly influence the intensity of an SEP event when 1AU observations of in-situ shocks clearly show that weak, slow shocks are inefficient accelerators of particles? How is a "preceding" CME identified, e.g., what is the maximum interval from the main CME, should it be in the same region of the sky, from the same active region? Some of these points will be discussed in the light of a large sample of ~500 SEP events including ~25 MeV protons observed by the STEREO or near-Earth spacecraft and during cycles 23 and 24 and the concurrent CME observations.