THE TWIN-CME SCENARIO AND LARGE SOLAR ENERGETIC PARTICLE EVENTS IN SOLAR CYCLE 23

Energetic particles in large solar energetic particle (SEP) events are a major concern for space weather. Recently, Li et al. proposed a twin-CME scenario for ground-level events. Here we extend that study to large SEP events in solar cycle 23. Depending on whether preceding coronal mass ejections (CMEs) within 9 hr exist and whether ions >10 MeV nucleon(-1) exceed 10 pfu, we categorize fast CMEs with speed >900 km s(-1) and width >60 degrees from the western hemisphere source regions into four groups: groups I and II are twin and single CMEs that lead to large SEPs; groups III and IV are twin and single CMEs that do not lead to large SEPs. The major findings of this paper are: first, large SEP events tend to be twin-CME events. Of 59 western large SEP events in solar cycle 23, 43 are twin-CME (group I) events and 16 are single-CME (group II) events. Second, not all twin CMEs produced large SEPs: 28 twin CMEs did not produce large SEPs (group III events). Some of them produced excesses of particles up to a few MeV nucleon(-1). Third, there were 39 single fast CMEs that did not produce SEPs (group IV events). Some of these also showed an excess of particles up to a few MeV nucleon(-1). For all four groups of events, we perform statistical analyses on properties such as the angular width, the speed, the existence of accompanying metric type II radio bursts, and the associated flare class for the main CMEs and the preceding CMEs.