Riquelme, M; Quataert, E; Verscharen, D; (2018) PIC Simulations of Velocity-space Instabilities in a Decreasing Magnetic Field: Viscosity and Thermal Conduction. The Astrophysical Journal , 854 (2) , Article 132. 10.3847/1538-4357/aaa6d1.

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Abstract

We use particle-in-cell (PIC) simulations of a collisionless, electron–ion plasma with a decreasing background magnetic field, B, to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If |B| decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with p_{||,j} > p_{\perp,j} (p_{||,j} and p_{\perp,j} represent the pressure of species j (electron or ion) parallel and perpendicular to B). Linear theory indicates that, for sufficiently large anisotropies, different velocity-space instabilities can be triggered. These instabilities in principle have the ability to pitch-angle scatter the particles, limiting the growth of the anisotropies. Our simulations focus on the nonlinear, saturated regime of the instabilities. This is done through the permanent decrease of |B| by an imposed plasma shear. We show that, in the regime 2 \lesssim \beta_j \lesssim 20 (\beta_j \equiv 8\pi p_j/B^2), the saturated ion and electron pressure anisotropies are controlled by the combined effect of the oblique ion firehose and the fast magnetosonic/whistler instabilities. These instabilities grow preferentially on the scale of the ion Larmor radius, and make \Delta p_e/p_{||,e} \approx \Delta p_i/p_{||,i} (where \Delta p_j=p_{\perp,j} - p_{||,j}). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons, {\lambda }_{e}, along the mean magnetic field, finding that {\lambda }_{e} depends strongly on whether |B| decreases or increases. Our results can be applied in studies of low-collisionality plasmas such as the solar wind, the intracluster medium, and some accretion disks around black holes.

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