PIC simulations of the effect of velocity space instabilities on electron viscosity and thermal conduction

Journal Article · Mon Jun 20 00:00:00 EDT 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/824/2/123· OSTI ID:22868976

Riquelme, Mario A. ^[1]; Quataert, Eliot ^[2]; Verscharen, Daniel ^[3]

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1. Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile (Chile)
2. Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720 (United States)
3. Space Science Center and Department of Physics, University of New Hampshire, Durham, NH 03824 (United States)

In low-collisionality plasmas, velocity-space instabilities are a key mechanism providing an effective collisionality for the plasma. We use particle-in-cell (PIC) simulations to study the interplay between electron- and ion-scale velocity-space instabilities and their effect on electron pressure anisotropy, viscous heating, and thermal conduction. The adiabatic invariance of the magnetic moment in low-collisionality plasmas leads to pressure anisotropy, Δp{sub j}≡p{sub ⊥,j}−p{sub ∥,j}>0, if the magnetic field B is amplified (p{sub ⊥,j} and p{sub ∥,j} denote the pressure of species j (electron, ion) perpendicular and parallel to B). If the resulting anisotropy is large enough, it can in turn trigger small-scale plasma instabilities. Our PIC simulations explore the nonlinear regime of the mirror, IC, and electron whistler instabilities, through continuous amplification of the magnetic field |B| by an imposed shear in the plasma. In the regime 1≲β{sub j}≲20 (β{sub j}≡8πp{sub j}/|B|{sup 2}), the saturated electron pressure anisotropy, Δp{sub e}/p{sub ∥,e}, is determined mainly by the (electron-lengthscale) whistler marginal stability condition, with a modest factor of ∼1.5–2 decrease due to the trapping of electrons into ion-lengthscale mirrors. We explicitly calculate the mean free path of the electrons and ions along the mean magnetic field and provide a simple physical prescription for the mean free path and thermal conductivity in low-collisionality β {sub j} ≳ 1 plasmas. Our results imply that velocity-space instabilities likely decrease the thermal conductivity of plasma in the outer parts of massive, hot, galaxy clusters. We also discuss the implications of our results for electron heating and thermal conduction in low-collisionality accretion flows onto black holes, including Sgr A* in the Galactic Center.

• OSTI ID: 22868976
• Journal Information: Astrophysical Journal, Vol. 824, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCRETION DISKS
ADIABATIC INVARIANCE
ANISOTROPY
BLACK HOLES
COMPUTERIZED SIMULATION
COSMIC ELECTRONS
GALAXY CLUSTERS
MAGNETIC FIELDS
MAGNETIC MOMENTS
MEAN FREE PATH
NONLINEAR PROBLEMS
PLASMA
SOLAR WIND
SPACE
THERMAL CONDUCTION
THERMAL CONDUCTIVITY
WHISTLER INSTABILITY