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Electron acceleration in solar flares and the transition from nonthermal to thermal hard X-ray phasesObservations are reviewed which indicate that hard X-rays during the impulsive phase of a flare typically start with a primarily nonthermal phase which undergoes a transition to a primarily thermal phase as the flare progresses. Recent theoretical work on the modified two-stream instability as an efficient electron accelerator and modeling of thermal hard X-ray sources is considered. A scenario which is termed the dissipative thermal model is proposed to explain the observations. Fast tearing modes occurring in a loop give rise to cross-field ion motion. This in turn excites the modified two-stream instability which converts about 50 percent of the ion energy into accelerated electrons along the loop as long as the plasma beta is less than 0.3. These electrons impact the chromosphere and boil off a part of it which rises up the loop. This density increase coupled with the temperature increase due to tearing causes the beta to increase beyond 0.3 and efficient electron acceleration ceases. This leads to the primarily thermal phase.
Document ID
19850042219
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Smith, D. F.
(Berkeley Research Associates Boulder, CO, United States)
Date Acquired
August 12, 2013
Publication Date
January 15, 1985
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 288
ISSN: 0004-637X
Subject Category
Solar Physics
Accession Number
85A24370
Funding Number(s)
CONTRACT_GRANT: NSF ATM-83-14511
CONTRACT_GRANT: NASW-3603
Distribution Limits
Public
Copyright
Other

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