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

Desai, M. I. and Mason, G. M. and Dayeh, M. A. and Ebert, R. W. and McComas, D. J. and Li, G. and Cohen, C. M. S. and Mewaldt, R. A. and Schwadron, N. A. and Smith, C. W. (2016) Spectral properties of large gradual solar energetic particle events II. Systematic Q/M dependence of heavy ion spectral breaks. Astrophysical Journal, 828 (2). Art. No. 106. ISSN 0004-637X. doi:10.3847/0004-637X/828/2/106.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Desai, M. I.0000-0002-7318-6008
Mason, G. M.0000-0003-2169-9618
Dayeh, M. A.0000-0001-9323-1200
Ebert, R. W.0000-0002-2504-4320
McComas, D. J.0000-0001-6160-1158
Li, G.0000-0001-9313-251X
Cohen, C. M. S.0000-0002-0978-8127
Mewaldt, R. A.0000-0003-2178-9111
Schwadron, N. A.0000-0002-3737-9283
Additional Information:© 2016 American Astronomical Society. Received 2016 January 19; revised 2016 June 2; accepted 2016 June 19; published 2016 September 8. We are grateful to the members of the Space Physics Group at the University of Maryland and the Johns Hopkins Applied Physics Laboratory (JHU/APL) for the construction of the ULEIS instrument and to members of Space Radiation Laboratory at the California Institute of Technology for the construction of the SIS instrument. We acknowledge use of the NOAA GOES and SOHO/ERNE proton data. Work at SwRI is partially supported by NASA grants NNX13AE07G and NNX13AI75G, NASA contracts NNX10AT75G and NNN06AA01C, and NSF Grants AGS-1135432 and AGS-1460118. Work at APL was supported by NASA grants NNX13AR20G/115828 and 44A-1091698.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:acceleration of particles; interplanetary medium; shock waves; solar wind; Sun: abundances; Sun: flares
Issue or Number:2
Record Number:CaltechAUTHORS:20160912-075708090
Persistent URL:
Official Citation:M. I. Desai et al 2016 ApJ 828 106
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70262
Deposited On:13 Sep 2016 23:09
Last Modified:11 Nov 2021 04:26

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