12-20 July 2017
Asia/Seoul timezone
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BEXCO - Room C(109/110)

[SH041] Spectral Properties of Large Gradual Solar Energetic Particle Events and Their Implications for near-Sun CME Shocks


  • Mihir DESAI

Primary authors


We fit the 0.1-500 MeV/nucleon H-Fe spectra in 46 large SEP events observed at 1 AU with the double power-law Band function to obtain a normalization constant, low- and high-energy parameters parameters $gamma_{a}$ and $gamma_{b}$, and break energy $E_B$, and derive the low-energy spectral slope $gamma_{1}$. We find that: 1) $gamma_{a}$, $gamma_{1}$, and $gamma_{b}$ are species-independent and the spectra steepen with increasing energy; 2) $E_B$'s decrease systematically with decreasing $Q/M$ scaling as $(Q/M)^{alpha}$; 3) $alpha$ varies between $sim$0.2-3 and is well correlated with the $sim$0.16-0.23 MeV/nucleon Fe/O; 4) In most events, $alphale$1.4, $gamma_{b}$-$gamma_{a}$>3, and the O $E_B$ increases with $gamma_{b}$-$gamma_{a}$; and 5) In many extreme events (associated with faster CMEs and GLEs), Fe/O and $^3$He/$^4$He ratios are enriched, $alphageq$1.4, $gamma_{b}$-$gamma_{a}$<3, and $E_B$ decreases with $gamma_{b}$-$gamma_{a}$. We suggest that the species-independence of $gamma_{a}$, $gamma_{1}$, and $gamma_{b}$, the $Q/M$ dependence of $E_B$ within an event, and the range of $alpha$ values occur due to diffusive acceleration by near-Sun CME shocks rather than due to direct flare contributions or due to scattering in interplanetary turbulence. We use the observed spectral properties to infer the strength, geometry, and turbulence properties of near-Sun CME shocks. For instance, using $gamma_{1}$ we infer that the average compression ratio for 33 near-Sun CME shocks is 2.49$pm$0.08. In most events, the $Q/M$-dependence of $E_B$ is consistent with the equal diffusion coefficient condition and the variability in $alpha$ 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 extreme events, enhanced wave power enables faster CME shocks to accelerate impulsive suprathermal ions more efficiently than ambient coronal ions. Finally, we discuss implications of these new results for theoretical models, how upcoming missions like the Solar Probe Plus and Solar Orbiter will help unravel the roles of injection, acceleration and transport in producing the Q/M-dependent SEP spectral breaks.