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Title: Electron strahl and halo formation in the solar wind

Abstract

Here, we propose a kinetic model describing the formation of the strahl and halo electron populations in the solar wind. We demonstrate that the suprathermal electrons propagating from the Sun along the Parker-spiral magnetic field lines are progressively focused into a narrow strahl at heliospheric distances r ≲ 1 au, while at r ≳ 1 au the width of the strahl saturates due to Coulomb collisions and becomes independent of the distance. Our theory of the strahl broadening does not contain free parameters and it agrees with Wind observations of the strahl width at 1 au to within |$$15\hbox{--}20{{\ \rm per\ cent}}$$|, for widths that are resolvable by the instrument. This indicates that Coulomb scattering, rather than anomalous turbulent diffusion, plays a dominant role in strahl formation in these observations. We further propose that the halo electron population at energies |$$K \lesssim 200\, \, {\rm eV}$$| may be composed of electrons that ran away from the Sun as an electron strahl, but later ended up on magnetic field lines leading them back to the Sun. The halo electrons are therefore not produced locally; rather, they are the fast electrons trapped by magnetic field lines on global heliospheric scales. Through the effects of magnetic defocusing and Coulomb pitch-angle scattering, a narrow source distribution at large heliocentric distances appears nearly isotropic at distances ~1 au. At larger energies |$$K \gtrsim 200\, \, {\rm eV}$$|, however, our theory indicates that the scattering provided by Coulomb collisions alone is not sufficient to isotropize a narrow sunward-propagating electron beam.

Authors:
ORCiD logo [1]; ORCiD logo [2];  [3]
  1. Department of Physics, University of Wisconsin – Madison, 1150 University Avenue, Madison, WI 53706, USA
  2. Department of Physics, University of Wisconsin – Madison, 1150 University Avenue, Madison, WI 53706, USA, Space Science Institute, Boulder, CO 80301, USA
  3. Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA, Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Publication Date:
Research Org.:
Univ. of Kansas, Lawrence, KS (United States); Univ. of Wisconsin, Madison, WI (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Science Foundation (NSF); National Aeronautic and Space Administration (NASA)
OSTI Identifier:
1492907
Alternate Identifier(s):
OSTI ID: 1612501
Grant/Contract Number:  
SC0018266; SC0016368; SC0019474; PHY-1707272; 80NSSC18K0640
Resource Type:
Published Article
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Name: Monthly Notices of the Royal Astronomical Society Journal Volume: 484 Journal Issue: 2; Journal ID: ISSN 0035-8711
Publisher:
Royal Astronomical Society
Country of Publication:
United Kingdom
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; astronomy & astrophysics; plasmas, Sun: heliosphere, solar wind

Citation Formats

Horaites, Konstantinos, Boldyrev, Stanislav, and Medvedev, Mikhail V. Electron strahl and halo formation in the solar wind. United Kingdom: N. p., 2018. Web. doi:10.1093/mnras/sty3504.
Horaites, Konstantinos, Boldyrev, Stanislav, & Medvedev, Mikhail V. Electron strahl and halo formation in the solar wind. United Kingdom. doi:https://doi.org/10.1093/mnras/sty3504
Horaites, Konstantinos, Boldyrev, Stanislav, and Medvedev, Mikhail V. Sat . "Electron strahl and halo formation in the solar wind". United Kingdom. doi:https://doi.org/10.1093/mnras/sty3504.
@article{osti_1492907,
title = {Electron strahl and halo formation in the solar wind},
author = {Horaites, Konstantinos and Boldyrev, Stanislav and Medvedev, Mikhail V.},
abstractNote = {Here, we propose a kinetic model describing the formation of the strahl and halo electron populations in the solar wind. We demonstrate that the suprathermal electrons propagating from the Sun along the Parker-spiral magnetic field lines are progressively focused into a narrow strahl at heliospheric distances r ≲ 1 au, while at r ≳ 1 au the width of the strahl saturates due to Coulomb collisions and becomes independent of the distance. Our theory of the strahl broadening does not contain free parameters and it agrees with Wind observations of the strahl width at 1 au to within |$15\hbox{--}20{{\ \rm per\ cent}}$|, for widths that are resolvable by the instrument. This indicates that Coulomb scattering, rather than anomalous turbulent diffusion, plays a dominant role in strahl formation in these observations. We further propose that the halo electron population at energies |$K \lesssim 200\, \, {\rm eV}$| may be composed of electrons that ran away from the Sun as an electron strahl, but later ended up on magnetic field lines leading them back to the Sun. The halo electrons are therefore not produced locally; rather, they are the fast electrons trapped by magnetic field lines on global heliospheric scales. Through the effects of magnetic defocusing and Coulomb pitch-angle scattering, a narrow source distribution at large heliocentric distances appears nearly isotropic at distances ~1 au. At larger energies |$K \gtrsim 200\, \, {\rm eV}$|, however, our theory indicates that the scattering provided by Coulomb collisions alone is not sufficient to isotropize a narrow sunward-propagating electron beam.},
doi = {10.1093/mnras/sty3504},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 2,
volume = 484,
place = {United Kingdom},
year = {2018},
month = {12}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
DOI: https://doi.org/10.1093/mnras/sty3504

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Cited by: 7 works
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