Charge-to-mass dependence of heavy ion spectral breaks in large gradual solar energetic particle events

Abstract

We fit the ∼0.1-500 MeV/nucleon H-Fe spectra in 46 large SEP events surveyed by [1] with the double power-law Band function to obtain a normalization constant, low- and high-energy parameters γa and γb ; and break energy EB . We also calculate the low-energy power-law spectral slope γ1. We find that: 1) γa , γ1, and γb are species-independent within a given SEP event, and the spectra steepen with increasing energy; 2) EB 's are well ordered by Q/M ratio, and decrease systematically with decreasing Q/M, scaling as (Q/M)α with α varying between ∼0.2-3; 3) α is well correlated with Fe/O at ∼0.16-0.23 MeV/nucleon and CME speed; 4) In most events: α <1.4 and the spectra steepen significantly at higher energy with γb -γa >3; and 5) Seven out of nine extreme SEP events (associated with faster CMEs and GLEs) are Fe-rich and have α >1.4 with flat spectra at low and high energies yielding γb -γa <3. The species-independence of γa , γ1, and γb and the systematic Q/M dependence of EB within an event, as well as the range of values for α suggest that the formation of double power-laws in SEP events occurs primarily due to diffusive acceleration at near-Sun CME shocks and not due to scattering in the interplanetary turbulence. In most events, the Q/M-dependence of EB is consistent with the equal diffusion coefficient condition while the event-to-event variations in a are probably driven by differences in the near-shock wave intensity spectra, which are flatter than the Kolmogorov turbulence spectrum but still weaker compared to that inferred for the extreme events. The weaker turbulence allows SEPs to escape more easily, resulting in weaker Q/M-dependence of EB , (lower a values) and spectral steepening at higher energies. In extreme events, the flatter spectra at high- and low-energy and stronger Q/M-dependence of EB (larger α values) occur due to enhanced wave power, which also enables the faster CME shocks to accelerate flare suprathermals more efficiently than ambient coronal ions.