Is the enhancement of type II radio bursts during CME interactions related to the associated solar energetic particle event?

We investigated 64 pairs of interacting-CME events identified from simultaneous observations by the SOHO and STEREO spacecraft from January 2010 to August 2014, to examine the relationship between large SEP events in the energy range of similar to 25 to similar to 60 MeV and properties of the interacting CMEs. We found that during CME, interactions, the large SEP events in this study were all generated by CMEs with the presence of enhanced type II radio bursts, which also have wider longitudinal distributions compared to events without a type II radio burst or its enhancement (almost always associated with small SEP events). It seems that the signature of type II radio burst enhancement is a good discriminator between large SEP and small or no SEP event producers during CME interactions. The type II radio burst enhancement is more likely to be generated by CME interactions, with the main CME having a larger speed (v), angular width (WD), mass (m) and kinetic energy (E-k), and taking over the preceding CMEs. The preceding CMEs in these instances have higher v, WD, in and E-k than those in CME, pairs missing type II radio bursts or enhancements. Generally, the values of these properties in the type-II-enhanced events are typically higher than the corresponding non-type-II or non-type-II-enhanced cases for both the main and preceding CMEs. Our analysis also revealed that the intensities of associated SEP events correlate negatively with the intersection height of the two CMEs. Moreover, the overlap width of two CMEs is typically larger in type-II-enhanced events than in non-type-II or non-type-II-enhanced events. Most type-II-enhanced events and SEP events are coincident and are almost always made by the fast and wide main CMEs that sweep fully over relatively slower and narrower preceding CMEs. We suggest that a fast CME with enough energy completely overtaking a relatively narrower preceding CME, especially at low height, can drive a more energetic shock signified by the enhanced type II radio bursts. The shock may accelerate ambient particles (likely provided by the preceding CME) and lead to large SEP events more easily.