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

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) ≤ 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 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, agreeing with earlier results by Reames (2009b).

What is Causing the Deficit of High-Energy Solar Particles in Solar Cycle 24?

“…We suggest that another key factor is the non-linear nature of CME-shock acceleration. It has been claimed that the largest SEP events occur when a fast CME follows on the heels of an earlier one from the same active region [19,20]. Although there may also be other factors at play, the seed particles generated by the first CME-shock provide a natural means of improving the acceleration efficiency at the second shock.…”

Tests for coronal electron temperature signatures in suprathermal electron populations at 1 AU

“…If suprathermal electrons are accelerated by ICME shocks in the corona in a similar manner to suprathermal ions (e.g. Kahler and Vourlidas, 2014;Ding et al, 2015), then this would represent a severe deviation from the scenario described in Sect. 1.…”

Statistical Characteristics on SEPs, Radio-Loud CMEs, Low Frequency Type II and Type III Radio Bursts Associated with Impulsive and Gradual Flares

“…Only half of the events were associated with meter wavelength type III radio bursts. Furthermore, many authors suggested that the ambient suprathermal seed particles from the previous solar flares [54] [12] [16] and from the preceding CMEs [23] [48] [16] [45] [58] can play an important role for producing the large SEP events.…”