Seed Population in Large Solar Energetic Particle Events and the Twin-CME Scenario

Abstract

It has recently been suggested that large solar energetic particle (SEP) events are often caused by twin coronal mass ejections (CMEs). In the twin-CME scenario, the preceding CME provides both an enhanced turbulence level and enhanced seed population at the main CME-driven shock. In this work, we study the effect of the preceding CMEs on the seed population. We examine event-integrated abundance of iron to oxygen ratio (Fe/O) at energies above 25 MeV/nuc for large SEP events in solar cycle 23. We find that the Fe/O ratio (normalized to the reference coronal value of 0.134) ≤2.0 for almost all single-CME events and these events tend to have smaller peak intensities. In comparison, the Fe/O ratio of twin-CME events scatters in a larger range, reaching as high as 8, suggesting the presence of flare material from perhaps preceding flares. For extremely large SEP events with peak intensities above 1000 pfu, the Fe/O ratios drop below 2, indicating that the seed particles are dominated by coronal material rather than flare material in these extreme events. The Fe/O ratios of ground level enhancement (GLE) events, which are all twin-CME events, scatter in a broad range. For a given Fe/O ratio, GLE events tend to have larger peak intensities than non-GLE events. Using velocity dispersion analysis, we find that GLE events have lower solar particle release heights than non-GLE events, agreeing with earlier results by Reames.