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

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, $p_{\perp,j} > p_{||,j}$, if the magnetic field $\vec{B}$ is amplified ($p_{\perp,j}$ and $p_{||,j}$ denote the pressure of species $j$ [electron, ion] perpendicular and parallel to $\vec{B}$). 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, ion-cyclotron, and electron whistler instabilities, through continuous amplification of the magnetic field $|\vec{B}|$ by an imposed shear in the plasma. In the regime $1 \lesssim \beta_j \lesssim 20$ ($\beta_j \equiv 8\pi p_j/|\vec{B}|^2$), the saturated electron pressure anisotropy, $\Delta p_e/p_{||,e}$, is determined mainly by the (electron-lengthscale) whistler marginal stability condition, with a modest factor of $\sim 1.5-2$ decrease due to the trapping of electrons by the 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 $\beta_j \gtrsim 1$ 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.