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

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) Charge-to-mass dependence of heavy ion spectral breaks in large gradual solar energetic particle events. Journal of Physics: Conference Series, 767 . Art. No. 012004. ISSN 1742-6596.

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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 E_B . 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) E_B '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 E_B , (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 E_B (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.

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Mason, G. M.0000-0003-2169-9618
Cohen, C. M. S.0000-0002-0978-8127
Additional Information:© 2016 IOP Publishing Ltd. Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 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 the 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
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Record Number:CaltechAUTHORS:20161129-081111412
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Official Citation:M. I. Desai et al 2016 J. Phys.: Conf. Ser. 767 012004
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:72362
Deposited By: Ruth Sustaita
Deposited On:29 Nov 2016 17:05
Last Modified:11 Sep 2017 18:14

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