The formation of kappa-distribution accelerated electron populations in solar flares

Bian, Nicolas H.; Stackhouse, Duncan J.; Kontar, Eduard P.

doi:10.1088/0004-637X/796/2/142

Title: The formation of kappa-distribution accelerated electron populations in solar flares

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Abstract

Driven by recent RHESSI observations of confined loop-top hard X-ray sources in solar flares, we consider stochastic acceleration of electrons in the presence of Coulomb collisions. If electron escape from the acceleration region can be neglected, the electron distribution function is determined by a balance between diffusive acceleration and collisions. Such a scenario admits a stationary solution for the electron distribution function that takes the form of a kappa distribution. We show that the evolution toward this kappa distribution involves a 'wave front' propagating forward in velocity space, so that electrons of higher energy are accelerated later; the acceleration timescales with energy according to τ{sub acc} ∼ E {sup 3/2}. At sufficiently high energies escape from the finite-length acceleration region will eventually dominate. For such energies, the electron velocity distribution function is obtained by solving a time-dependent Fokker-Planck equation in the 'leaky-box' approximation. Solutions are obtained in the limit of a small escape rate from an acceleration region that can effectively be considered a thick target.

Authors:

Bian, Nicolas H.; Stackhouse, Duncan J.; Kontar, Eduard P. ^[1]

School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ (United Kingdom)

Publication Date:: Mon Dec 01 00:00:00 EST 2014

OSTI Identifier:: 22370125

Resource Type:: Journal Article

Journal Name:: Astrophysical Journal

Additional Journal Information:: Journal Volume: 796; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X

Country of Publication:: United States

Language:: English

Subject:: 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; APPROXIMATIONS; COSMIC X-RAY SOURCES; DISTRIBUTION; DISTRIBUTION FUNCTIONS; ELECTRONS; FOKKER-PLANCK EQUATION; GAMMA RADIATION; HARD X RADIATION; SOLAR FLARES; SPACE; STAR EVOLUTION; STOCHASTIC PROCESSES; SUN; TIME DEPENDENCE; VELOCITY

Citation Formats


                    Bian, Nicolas H., Stackhouse, Duncan J., Kontar, Eduard P., and Emslie, A. Gordon, E-mail: n.bian@physics.gla.ac.uk, E-mail: d.stackhouse.1@research.gla.ac.uk, E-mail: eduard@astro.gla.ac.uk, E-mail: emslieg@wku.edu. The formation of kappa-distribution accelerated electron populations in solar flares.  United States: N. p., 2014. 
        Web.  doi:10.1088/0004-637X/796/2/142.

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                    Bian, Nicolas H., Stackhouse, Duncan J., Kontar, Eduard P., & Emslie, A. Gordon, E-mail: n.bian@physics.gla.ac.uk, E-mail: d.stackhouse.1@research.gla.ac.uk, E-mail: eduard@astro.gla.ac.uk, E-mail: emslieg@wku.edu. The formation of kappa-distribution accelerated electron populations in solar flares.  United States.  https://doi.org/10.1088/0004-637X/796/2/142

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                    Bian, Nicolas H., Stackhouse, Duncan J., Kontar, Eduard P., and Emslie, A. Gordon, E-mail: n.bian@physics.gla.ac.uk, E-mail: d.stackhouse.1@research.gla.ac.uk, E-mail: eduard@astro.gla.ac.uk, E-mail: emslieg@wku.edu. 2014.  
        "The formation of kappa-distribution accelerated electron populations in solar flares".  United States.  https://doi.org/10.1088/0004-637X/796/2/142.

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@article{osti_22370125,

  title        = {The formation of kappa-distribution accelerated electron populations in solar flares},

  author       = {Bian, Nicolas H. and Stackhouse, Duncan J. and Kontar, Eduard P. and Emslie, A. Gordon, E-mail: n.bian@physics.gla.ac.uk, E-mail: d.stackhouse.1@research.gla.ac.uk, E-mail: eduard@astro.gla.ac.uk, E-mail: emslieg@wku.edu},

  abstractNote = {Driven by recent RHESSI observations of confined loop-top hard X-ray sources in solar flares, we consider stochastic acceleration of electrons in the presence of Coulomb collisions. If electron escape from the acceleration region can be neglected, the electron distribution function is determined by a balance between diffusive acceleration and collisions. Such a scenario admits a stationary solution for the electron distribution function that takes the form of a kappa distribution. We show that the evolution toward this kappa distribution involves a 'wave front' propagating forward in velocity space, so that electrons of higher energy are accelerated later; the acceleration timescales with energy according to τ{sub acc} ∼ E {sup 3/2}. At sufficiently high energies escape from the finite-length acceleration region will eventually dominate. For such energies, the electron velocity distribution function is obtained by solving a time-dependent Fokker-Planck equation in the 'leaky-box' approximation. Solutions are obtained in the limit of a small escape rate from an acceleration region that can effectively be considered a thick target.},

  doi          = {10.1088/0004-637X/796/2/142},

  url          = {https://www.osti.gov/biblio/22370125},
  journal      = {Astrophysical Journal},

  issn         = {0004-637X},

  number       = 2,

  volume       = 796,

  place        = {United States},

  year         = {Mon Dec 01 00:00:00 EST 2014},

  month        = {Mon Dec 01 00:00:00 EST 2014}

}

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