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Characteristics of Solar Energetic Ions as a Function of Longitude

Cohen, C. M. S. and Mason, G. M. and Mewaldt, R. A. (2017) Characteristics of Solar Energetic Ions as a Function of Longitude. Astrophysical Journal, 843 (2). Art. No. 132. ISSN 1538-4357.

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Since the 2006 launch of STEREO, multi-spacecraft studies have yielded several surprising results regarding the spread of solar energetic particles (SEPs) within the inner heliosphere. We have investigated the role of energy and ridigity, using ACE and STEREO 10 MeV n^(−1) oxygen data to identify 41 large SEP events observed by two or three spacecraft. We calculated fluence spectra from ~0.1 to >10 MeV n^(−1) for H, He, O, and Fe for each event at the observing spacecraft (including SOHO and GOES). The particle fluences at 0.3, 1, and 10 MeV n^(−1) were examined as a function of the distance between the associated solar flare longitude and the spacecraft magnetic footpoints at the Sun to determine the longitudinal spread of particles and study how the distribution centers and widths depend on energy and charge-to-mass (Q/M) for the first time. On average, the three-spacecraft event distributions were centered at 22 ± 4° west of the flare site and were 43 ± 1° wide, though there was substantial variability, while the fit to the aggregate of the two-spacecraft event fluences yielded significantly wider distributions at 0.3 and 1 MeV n^(−1). The widths derived from both the three- and two-spacecraft events show an energy dependence with distributions narrowing with increasing energy, consistent with lower energy ions experiencing more field line co-rotation, or being accelerated over a larger portion of the CME-driven shock or for longer times as the shock expands. Surprisingly, no clear evidence was found for a Q/M dependence to the widths or centers suggesting that rigidity-related processes are not the dominant means of spreading particles in longitude.

Item Type:Article
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Cohen, C. M. S.0000-0002-0978-8127
Mason, G. M.0000-0003-2169-9618
Additional Information:© 2017 American Astronomical Society. Received 2017 March 23. Accepted 2017 May 23. Published 2017 July 13. The authors thank the STEREO/SEPT, STEREO/SECCHI, STEREO/WAVES, SOHO/EPHIN, Wind/WAVES, and SDO/AIA teams for making their data publicly available, Predicitve Science Inc. for their publicly available model-based field line tracing, L. Jian for her STEREO ICME list, and I. Richardson and H. Cane for their near-Earth ICME list. This work was supported by NSF grants 1156004 and 1156138 and NASA grants NNX11075G, NNX13AH66G, and NNX15AG09G. Work at Johns Hopkins University APL was supported by NASA grant NNX17AC05G/125225, and University of California Berkeley grant 00008934. This paper uses data from the CACTus CME catalog, generated and maintained by the SIDC at the Royal Observatory of Belgium; the Heliospheric Shock Database, generated and maintained at the University of Helsinki; and the SOHO CME catalog, generated and maintained at the CDAW Data Center by NASA and the Catholic University of America in cooperation with the Naval Research Laboratory (SOHO is a project of international cooperation between ESA and NASA). The GOES 8-15 particle data are produced in real time by the NOAA Space Weather Prediction Center (SWPC) and are distributed by the NOAA National Geophysical Data Center (NGDC).
Group:Space Radiation Laboratory
Funding AgencyGrant Number
University of California Berkeley00008934
Subject Keywords:acceleration of particles; interplanetary medium; solar–terrestrial relations; Sun: coronal mass ejections, (CMEs) ; Sun: flares; Sun: particle emission
Record Number:CaltechAUTHORS:20170713-101353720
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Official Citation:C. M. S. Cohen et al 2017 ApJ 843 132
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:79077
Deposited By: Ruth Sustaita
Deposited On:13 Jul 2017 18:50
Last Modified:13 Jul 2017 18:50

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