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PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction

Riquelme, MA; Quataert, E; Verscharen, D; (2016) PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction. Astrophysical Journal , 824 (2) , Article 123. 10.3847/0004-637X/824/2/123. Green open access

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Abstract

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, ${\rm{\Delta }}{p}_{j}\equiv {p}_{\perp ,j}-{p}_{\parallel ,j}\gt 0$, if the magnetic field ${\boldsymbol{B}}$ is amplified (${p}_{\perp ,j}$ and ${p}_{\parallel ,j}$ denote the pressure of species j (electron, ion) perpendicular and parallel to ${\boldsymbol{B}}$). 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 $| {\boldsymbol{B}}| $ by an imposed shear in the plasma. In the regime $1\lesssim {\beta }_{j}\lesssim 20$ (${\beta }_{j}\equiv 8\pi {p}_{j}/| {\boldsymbol{B}}{| }^{2}$), the saturated electron pressure anisotropy, ${\rm{\Delta }}{p}_{{\rm{e}}}/{p}_{\parallel ,{\rm{e}}}$, 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 j gsim 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.

Type: Article
Title: PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction
Open access status: An open access version is available from UCL Discovery
DOI: 10.3847/0004-637X/824/2/123
Publisher version: https://doi.org/10.3847/0004-637X/824/2/123
Language: English
Additional information: This is the published version of record. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: accretion, accretion disks, instabilities, plasmas, solar wind
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/1575122
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