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

TL;DR

This study investigates how twin CMEs influence seed particle populations in large SEP events, revealing that twin CMEs often have higher Fe/O ratios and lower SPR heights, affecting event intensity and composition.

Contribution

It provides new insights into the role of preceding CMEs in shaping seed populations and their impact on SEP event characteristics, especially in extreme and GLE events.

Findings

Twin-CME events show higher Fe/O ratios than single-CME events.

Extreme SEP events with peak intensity >1000 pfu have Fe/O < 2.

GLE events have lower SPR heights and larger peak intensities than non-GLE events.

Abstract

It has been recently suggested that large solar energetic particle (SEP) events are often caused by twin CMEs. In the twin-CME scenario, the preceding CME is to provide 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 $0.134$) $\leq2.0$ 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 intensity above $1000$ pfu, the Fe/O drop below $2$, indicating that in these extreme events the seed particles are dominated by coronal material than flare material. The Fe/O ratios of Ground level enhancement (GLE) events, all being 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 (VDA), we find that GLE events have lower solar particle release (SPR) heights than non-GLE events, \red{agreeing with earlier results by Reames 2009b.