Q/M-Dependence of Heavy Ion Spectral Breaks in Large SEP events
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Left panel: Event-integrated differential fluences versus energy of ~0.1-500 MeV per nucleon H-Fe nuclei (offset for clarity) with Band-function fits (solid lines) during a large gradual SEP event that also produced a ground level enhancement (GLE). Data are from ACE/EPAM, ACE/ULEIS, SoHO/ERNE, GOES/EPS, SAMPEX/PET, and ACE/SIS. Middle panel: Break energy E_X of species X normalized to the proton spectral break energy E_H vs. the ion's charge-to-mass (Q/M) ratio. Solid line is the fit to the data with the linear function log(E_X/E_H) = log(n₀) + α log(Q_X/M_X); dashed line is α=0.2; dotted line is α=2. Right panel: Distribution of α in 33 large gradual SEP events.

Heavy-ion energy spectra in large gradual solar energetic particle (SEP) events associated with coronal mass ejections (CMEs) are often represented by two broken power-laws with distinct low and high-energy spectral slopes. We fit the ~0.1-500 MeV per nucleon H-Fe spectra in 46 large 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 spectral break energy E_B, and derive the low-energy spectral slope γ₁. We use the power-law slopes and the location of the transition or break energies to infer the seed populations and conditions under which the SEPs are accelerated by near-Sun CME shocks.

Our results show that: 1) γ_a, γ₁, and γ_b are species-independent and the spectra steepen with increasing energy (left panel); 2) E_B's decrease systematically with decreasing Q/M scaling as (Q/M)^α (middle panel); 3) In 33 events, α varies between ~0.2-3 (right panel) and is well correlated with the ~0.16-0.23 MeV per nucleon Fe/O; 4) In most events, α < 1.4, γ_b - γ_a > 3, and the O E_B increases with γ_b - γ_a; and 5) In many extreme events (associated with faster CMEs and GLEs), the 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, γ₁ 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 γ₁ 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. For further details see, Desai et al., Astrophysical Journal, Vol. 828, 106, 2016;doi:10.3847/0004-637X/828/2/106.

This item was contributed by M. I. Desai, M. A. Dayeh and R. W. Ebert of the Southwest Research Institute, G. M. Mason of the Johns Hopkins University Applied Physics Laboratory, D. J. McComas of Princeton University, G. Li of the University of Alabama in Huntsville, C. M. S. Cohen and R. A. Mewaldt of Caltech, and N. A. Schwadron and C. W. Smith of the University of New Hampshire. Address questions and comments to

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Last modified 22 September 2016.