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Spectral Properties of Large Gradual Solar Energetic Particle Events. I. Fe, O, and Seed Material

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. I. Fe, O, and Seed Material. Astrophysical Journal, 816 (2). Art. No. 68. ISSN 0004-637X. doi:10.3847/0004-637X/816/2/68.

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We have surveyed ~0.1–100 MeV nucleon^(−1) O and Fe fluence spectra during 46 isolated, large gradual SEP events observed at ACE during solar cycles 23 and 24. Most SEP spectra are well represented by the four-parameter Band function with a normalization constant, low-energy spectral slope, high-energy spectral slope, and break energy. The O and Fe spectral slopes are similar and most spectra steepen above the break energy, probably due to common acceleration and transport processes affecting different ion species. SEP spectra above the break energies depend on the origin of the seed population; larger contributions of suprathermal flare material result in higher Fe/O ratios and flatter spectra at higher energies. SEP events with steeper O spectra at low energies and higher break energies are associated with slower coronal mass ejections (CMEs), while those associated with fast (>2000 km s^(−1)) CMEs and ground level enhancements have harder or flatter spectra at low and high energies, and O break energies between ~1 and 10 MeV nucleon^(−1). The latter events are enriched in ^3He and higher-energy Fe, and have Fe spectra that rollover at significantly lower energies compared with O, probably because Fe ions with smaller Q/M ratios can escape from the distant shock more easily than O ions with larger Q/M ratios. We conclude that SEP spectral properties result from many complex and competing effects, namely Q/M-dependent scattering, shock properties, and the origin of the seed populations, all of which must be taken into account to develop a comprehensive picture of CME-driven shock acceleration of large gradual SEP events.

Item Type:Article
Related URLs:
URLURL TypeDescription
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. The American Astronomical Society. Received 2015 September 16; accepted 2015 November 4; published 2016 January 11. 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 SPE list. 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 and NSF grants AGS-1156138 and AGS-1460118.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:interplanetary medium; shock waves; Sun: abundances; Sun: coronal mass ejections (CMEs) ; Sun: particle emission
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2016-36
Issue or Number:2
Record Number:CaltechAUTHORS:20160205-111440346
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Official Citation:M. I. Desai et al 2016 ApJ 816 68
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64270
Deposited By: George Porter
Deposited On:08 Feb 2016 19:07
Last Modified:10 Nov 2021 23:28

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