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High-Energy Solar Particle Events in Cycle 24The Sun is already in the declining phase of cycle 24, but the paucity of high-energy solar energetic particle (SEP) events continues with only two ground level enhancement (GLE) events as of March 31, 2015. In an attempt to understand this, we considered all the large SEP events of cycle 24 that occurred until the end of 2014. We compared the properties of the associated CMEs with those in cycle 23. We found that the CME speeds in the sky plane were similar, but almost all those cycle-24 CMEs were halos. A significant fraction of (16%) of the frontside SEP events were associated with eruptive prominence events. CMEs associated with filament eruption events accelerate slowly and attain peak speeds beyond the typical GLE release heights. When we considered only western hemispheric events that had good connectivity to the CME nose, there were only 8 events that could be considered as GLE candidates. One turned out to be the first GLE event of cycle 24 (2012 May 17). In two events, the CMEs were very fast (>2000 km/s) but they were launched into a tenuous medium (high Alfven speed). In the remaining five events, the speeds were well below the typical GLE CME speed (~2000 km/s). Furthermore, the CMEs attained their peak speeds beyond the typical heights where GLE particles are released. We conclude that several factors contribute to the low rate of high-energy SEP events in cycle 24: (i) reduced efficiency of shock acceleration (weak heliospheric magnetic field), (ii) poor latitudinal and longitudinal connectivity), and (iii) variation in local ambient conditions (e.g., high Alfven speed).
Document ID
20160007919
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Gopalswamy, N.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Makela, P.
(Catholic Univ. of America Washington, DC, United States)
Yashiro, S.
(Catholic Univ. of America Washington, DC, United States)
Xie, H.
(Catholic Univ. of America Washington, DC, United States)
Akiyama, S.
(Catholic Univ. of America Washington, DC, United States)
Thakur, N.
(Catholic Univ. of America Washington, DC, United States)
Date Acquired
June 27, 2016
Publication Date
September 30, 2015
Publication Information
Publication: Journal of Physics: Conference Series
Publisher: IOP Publishing
Volume: 642
Issue: 1
e-ISSN: 1742-6596
Subject Category
Solar Physics
Report/Patent Number
GSFC-E-DAA-TN32035
Funding Number(s)
CONTRACT_GRANT: NNX15AB77G
CONTRACT_GRANT: NNG11PL10A
CONTRACT_GRANT: NNX15AB70G
Distribution Limits
Public
Copyright
Other
Keywords
Solar Cycle 24
Solar particle events

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