Spectral properties of large gradual solar energetic particle events II. Systematic Q/M dependence of heavy ion spectral breaks

Abstract

We fit ~0.1–500 MeV nucleon^(−1) H–Fe spectra in 46 large solar energetic particle (SEP) events with the double power-law Band function to obtain a normalization constant, low- and high-energy parameters γ_a and γ_b, and break energy E_ B, and derive the low-energy spectral slope γ_1. We find that: (1) γ_a, γ_1, and γ_b are species-independent and the spectra steepen with increasing energy; (2) E_B decreases systematically with decreasing Q/M scaling as (Q/M)^α ; (3) α varies between ~0.2–3 and is well correlated with the ~0.16–0.23 MeV nucleon^(−1) Fe/O; (4) in most events, α < 1.4, γ_b–γ_a > 3, and O E_B increases with γ_b–γ_a; and (5) in many extreme events (associated with faster coronal mass ejections (CMEs) and GLEs), Fe/O and ^3He/^4He ratios are enriched, α ⩾ 1.4, γ_b–γ_a < 3, and E B decreases with γ_b–γ_a. The species-independence of γ_a, γ_1, and γ_b and the Q/M dependence of E_B within an event and the α values suggest that double power-law SEP spectra occur due to diffusive acceleration by near-Sun CME shocks rather than scattering in interplanetary turbulence. Using γ_1, we infer that the average compression ratio for 33 near-Sun CME shocks is 2.49 ± 0.08. In most events, the Q/M dependence of E B is consistent with the equal diffusion coefficient condition and the variability in α is driven by differences in the near-shock wave intensity spectra, which are flatter than the Kolmogorov turbulence spectrum but weaker than the spectra for extreme events. In contrast, in extreme events, enhanced wave power enables faster CME shocks to accelerate impulsive suprathermal ions more efficiently than ambient coronal ions.

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