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It has been recently suggested that large solar energetic particle (SEP)events are often caused by twin CMEs. In the twin-CME scenario, the precedingCME is to provide both an enhanced turbulence level and enhanced seedpopulation at the main CME-driven shock. In this work, we study the effect ofthe preceding CMEs on the seed population. We examine event-integratedabundance of iron to oxygen ratio (Fe/O) at energies above 25 MeV/nuc for largeSEP events in solar cycle 23. We find that the Fe/O ratio (normalized to thereference coronal value of $0.134$) $\leq2.0$ for almost all single-CME eventsand these events tend to have smaller peak intensities. In comparison, the Fe/Oratio of twin-CME events scatters in a larger range, reaching as high as $8$,suggesting the presence of flare material from perhaps preceding flares. Forextremely large SEP events with peak intensity above $1000$ pfu, the Fe/O dropbelow $2$, indicating that in these extreme events the seed particles aredominated by coronal material than flare material. The Fe/O ratios of Groundlevel enhancement (GLE) events, all being twin-CME events, scatter in a broadrange. For a given Fe/O ratio, GLE events tend to have larger peak intensitiesthan non-GLE events. Using velocity dispersion analysis (VDA), we find that GLEevents have lower solar particle release (SPR) heights than non-GLE events,\red{agreeing with earlier results by Reames 2009b.

Seed population in large Solar Energetic Particle events and the twin-CME scenario