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:
-
- Department of Physics, University of Wisconsin – Madison, 1150 University Avenue, Madison, WI 53706, USA
- Department of Physics, University of Wisconsin – Madison, 1150 University Avenue, Madison, WI 53706, USA, Space Science Institute, Boulder, CO 80301, USA
- 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 Aeronautics 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. 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. 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 = {Sat Dec 29 00:00:00 EST 2018},
month = {Sat Dec 29 00:00:00 EST 2018}
}
https://doi.org/10.1093/mnras/sty3504
Web of Science
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