### Abstract

We derive the equations for cosmic-ray transport in a moving, scattering fluid in the presence of an average background magnetic field which is carried with the fluid. We proceed by expanding the cosmic-ray distribution function about momentum isotropy, keeping three terms. We identify these terms as the isotropic distribution, a streaming flux, and an anisotropic pressure tensor. Relating terms in the pressure tensor to spatial variations of the fluid flow velocity, we find five independent cosmic-ray viscosity coefficients. We present a general discussion of this viscosity and its relation to particle orbits in a magnetic field. Some effects correspond to simple extensions of viscous damping, modified by the magnetic field, but others, which change sign with the magnetic field or particle charge, represent finite gyroradius effects which do not contribute to the damping. We apply the equations to the cases of simple shear in the presence of a magnetic field, and not to the propagation of transverse Alfvén waves. One effect of viscosity which appears not to have been recognized previously is to rotate the polarization plane of a linearly polarized Alfvén wave.

Original language | English (US) |
---|---|

Pages (from-to) | 639-647 |

Number of pages | 9 |

Journal | Astrophysical Journal |

Volume | 371 |

Issue number | 2 |

State | Published - Apr 20 1991 |

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### Keywords

- Cosmic rays: general
- Hydromagnetics
- Particle acceleration
- Wave motions

### ASJC Scopus subject areas

- Space and Planetary Science

### Cite this

*Astrophysical Journal*,

*371*(2), 639-647.

**Viscosity and inertia in cosmic-ray transport : Effects of AN average magnetic field.** / Williams, L. L.; Jokipii, J. Randy.

Research output: Contribution to journal › Article

*Astrophysical Journal*, vol. 371, no. 2, pp. 639-647.

}

TY - JOUR

T1 - Viscosity and inertia in cosmic-ray transport

T2 - Effects of AN average magnetic field

AU - Williams, L. L.

AU - Jokipii, J. Randy

PY - 1991/4/20

Y1 - 1991/4/20

N2 - We derive the equations for cosmic-ray transport in a moving, scattering fluid in the presence of an average background magnetic field which is carried with the fluid. We proceed by expanding the cosmic-ray distribution function about momentum isotropy, keeping three terms. We identify these terms as the isotropic distribution, a streaming flux, and an anisotropic pressure tensor. Relating terms in the pressure tensor to spatial variations of the fluid flow velocity, we find five independent cosmic-ray viscosity coefficients. We present a general discussion of this viscosity and its relation to particle orbits in a magnetic field. Some effects correspond to simple extensions of viscous damping, modified by the magnetic field, but others, which change sign with the magnetic field or particle charge, represent finite gyroradius effects which do not contribute to the damping. We apply the equations to the cases of simple shear in the presence of a magnetic field, and not to the propagation of transverse Alfvén waves. One effect of viscosity which appears not to have been recognized previously is to rotate the polarization plane of a linearly polarized Alfvén wave.

AB - We derive the equations for cosmic-ray transport in a moving, scattering fluid in the presence of an average background magnetic field which is carried with the fluid. We proceed by expanding the cosmic-ray distribution function about momentum isotropy, keeping three terms. We identify these terms as the isotropic distribution, a streaming flux, and an anisotropic pressure tensor. Relating terms in the pressure tensor to spatial variations of the fluid flow velocity, we find five independent cosmic-ray viscosity coefficients. We present a general discussion of this viscosity and its relation to particle orbits in a magnetic field. Some effects correspond to simple extensions of viscous damping, modified by the magnetic field, but others, which change sign with the magnetic field or particle charge, represent finite gyroradius effects which do not contribute to the damping. We apply the equations to the cases of simple shear in the presence of a magnetic field, and not to the propagation of transverse Alfvén waves. One effect of viscosity which appears not to have been recognized previously is to rotate the polarization plane of a linearly polarized Alfvén wave.

KW - Cosmic rays: general

KW - Hydromagnetics

KW - Particle acceleration

KW - Wave motions

UR - http://www.scopus.com/inward/record.url?scp=0000666332&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0000666332&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:0000666332

VL - 371

SP - 639

EP - 647

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

ER -