Local re-acceleration and a modified thick target model of solar flare electrons

Brown, J.C., Turkmani, R., Kontar, E.P. , MacKinnon, A.L. and Vlahos, L. (2009) Local re-acceleration and a modified thick target model of solar flare electrons. Astronomy and Astrophysics, 508(2), pp. 993-1000. (doi: 10.1051/0004-6361/200913145)




<p><b>Context:</b> The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost “standard model” of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.</p> <p><b>Aims:</b> We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection. Methods. We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric () fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.</p> <p><b>Results:</b> Combined MHD and test particle simulations show that local fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfvén speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.</p> <p><b>Conclusions:</b> Including electron re-acceleration in the HXR source allows an LRTTM modification of the CTTM in which beam density and anisotropy are much reduced, and alleviates theoretical problems with the CTTM, while making it more compatible with radio and interplanetary electron numbers. The LRTTM is, however, different in some respects such as spatial distribution of atmospheric heating by fast electrons.</p>

Item Type:Articles
Glasgow Author(s) Enlighten ID:Kontar, Professor Eduard and Brown, Professor John and MacKinnon, Dr Alexander
Authors: Brown, J.C., Turkmani, R., Kontar, E.P., MacKinnon, A.L., and Vlahos, L.
College/School:College of Social Sciences > School of Education
Journal Name:Astronomy and Astrophysics
Publisher:EDP Sciences
ISSN (Online):1432-0746
Published Online:15 October 2009
Copyright Holders:Copyright © 2009 EDP Sciences
First Published:First published in Astronomy and Astrophysics 508(2):993-1000
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
398461Acceleration, propagation and emission of energetic particles from the sun to the earthEduard KontarParticle Physics & Astronomy Research Council (PPARC)PP/C001656/1Physics and Astronomy
465931Solar, stellar and cosmological plasmas: a synthesis of data, modelling and theory.Declan DiverScience & Technologies Facilities Council (STFC)ST/F002149/1Physics and Astronomy