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Investigating the Causes of Solar-Cycle Variations in Solar Energetic Particle Fluences and Composition

Mewaldt, R. A. and Cohen, C. M. S. and Mason, G. M. and von Rosenvinge, T. T. and Li, G. and Smith, C. W. and Vourlidas, A. (2015) Investigating the Causes of Solar-Cycle Variations in Solar Energetic Particle Fluences and Composition. In: 34th International Cosmic Ray Conference (ICRC2015). Proceedings of Science. No.236. SISSA , Trieste, Italy, Art. No. 030.

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Measurements with the ACE, STEREO, and GOES spacecraft during the first 5.8 years of solar cycle 24 show that the number of large Solar Energetic Particle (SEP) events is reduced by ~32% compared to this point of cycle 23, while the fluences of >10 MeV/nucleon ions from H to Ni are reduced by factors ranging from 4 to ~10. A comparison of H, O, and Fe energy spectra from the ten largest events of the two cycles shows that the spectral breaks that are typically observed in SEP energy spectra are occurring ~3 times lower in energy/nucleon than in cycle 23. We investigate the origin of these cycle-to-cycle fluence, spectral and composition differences by evaluating possible factors that include: 1) the properties of the associated CMEs; 2) the interplanetary magnetic field strength; and 3) the density of suprathermal seed particles. These properties are evaluated in the context of existing SEP acceleration models. We conclude that both the reduced magnetic field strength and the reduced seed particle densities are contributing to the reduction in SEP output during cycle 24. In particular, we point out that in the standard model for SEP shock acceleration the maximum energy achieved is a strong function of the rate at which protons are injected into the shock acceleration process.

Item Type:Book Section
Related URLs:
URLURL TypeDescription
Mewaldt, R. A.0000-0003-2178-9111
Cohen, C. M. S.0000-0002-0978-8127
Mason, G. M.0000-0003-2169-9618
Vourlidas, A.0000-0002-8164-5948
Alternate Title:An Investigation of the Causes of Solar-Cycle Variations in SEP Fluences and Composition
Additional Information:Copyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-ShareAlike. The work at Caltech was funded by NASA LWS TR&T grant NNX11A075G, NASA grant NNX13A66G, NASA contract NAS5-03131 from UC Berkeley, and by NSF grant 1156004. APL was supported by NASA grant NNX13AR20G/115828 and contract SA44889-26309 from UC Berkeley to JHU/APL. The University of Alabama/Huntsville was funded by NASA grant NNX15AJ93G. The University of New Hampshire was supported by NASA grant NNX13A66G. Participation by Goddard Space Flight Center was funded directly by NASA through its ACE, STEREO and LWS programs.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
University of California, BerkeleySA44889-26309
Series Name:Proceedings of Science
Issue or Number:236
Record Number:CaltechAUTHORS:20180910-131321667
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89497
Deposited By: George Porter
Deposited On:10 Sep 2018 23:22
Last Modified:09 Mar 2020 13:19

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