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ACE News #186 - September 22, 2016
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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 EX of species X
normalized to the proton spectral break energy EH vs. the ion's
charge-to-mass (Q/M) ratio. Solid line is the fit to the data with the linear
function log(EX/EH) =
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 γa and γb, and spectral break energy EB, 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) γa, γ1, and γb are species-independent and the spectra steepen with increasing energy (left panel); 2) EB'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, γb - γa > 3, and the O EB increases with γb - γa; and 5) In many extreme events (associated with faster CMEs and GLEs), the Fe/O and 3He/4He ratios are enriched, α ≥ 1.4, γb - γa < 3, and EB decreases with γb - γa.
The species-independence of γa, γ1 and γb and the Q/M dependence of EB 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 EB 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
Last modified 22 September 2016.