Ne/O and Fe/O Characteristics of Large Solar Energetic Particle Events during Solar Cycles 23 and 24

Abstract

Abstract We have examined the Ne/O and Fe/O characteristics of large solar energetic particle (SEP) events at the ion energy range of 3–40 MeV nucleon−1 during solar cycles 23 and 24. In each cycle, the solar activity displays an ∼3 yr rising phase and a longer declining phase. While Fe-poor events only appeared in the declining phase of cycle 23, the properties of Fe-rich events were similar in the rising phases of both cycles. Also, very few Fe-rich events were seen in the declining phase of cycle 24. In addition, the Ne/O data in the corona, solar wind, and SEP events consistently reveal that the characteristics of SEP events are mainly governed by the solar wind turbulence status that exhibits a significant difference between slow and fast streams. During the rising phase of the solar cycles, slow streams are dominated by the two-dimensional turbulence component, which significantly reduces the injection energy of the quasi-perpendicular (Q-Perp) shock acceleration. Also, slow streams have an increased Ne/O ratio and hence enhanced temperature of coronal suprathermals, favoring the occurrence of Fe-rich events. In contrast, in the declining phase of the solar cycles, the fast streams are dominated by the slab turbulence component, which could significantly increase the injection energy of the Q-Perp shock acceleration. Consequently, in fast streams, most Fe-rich events originate from jet suprathermals. The coronal suprathermals may produce the Fe-poor events having abnormally low Ne/O ratios provided the speed of the associated coronal mass ejection is large enough.