Abstract
Abstract We use particle-in-cell (PIC) simulations of a collisionless, electron–ion plasma with a decreasing background magnetic field, , to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with ( and 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 by an imposed plasma shear. We show that, in the regime ( ), 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 (where ). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons, , along the mean magnetic field, finding that depends strongly on whether 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.
Highlights
In low-collisionality plasmas, the change in the magnitude of the local magnetic field (B o ∣B∣) generically drives a pressure anisotropy with p,j 1 p^,j
The combined effect of pressure anisotropies and velocity-space instabilities can affect various large-scale properties of the plasma, including its effective viscosity (Sharma et al 2006; Squire et al 2017) and thermal conductivity. This weakly collisional behavior is expected to be important in several astrophysical systems, including lowluminosity accretion flows around compact objects (Sharma et al 2007), the intracluster medium (ICM) (Schekochihin et al 2005; Lyutikov 2007), and the heliosphere (Maruca et al 2011; Remya et al 2013)
We find that the thresholds of the oblique ion firehose (OIF) and fast magnetosonic/whistler (FM/W) modes with Dpe = Dpi continue to be similar and smaller than the oblique electron firehose (OEF) threshold by a factor ∼1.5
Summary
In low-collisionality plasmas, the change in the magnitude of the local magnetic field (B o ∣B∣) generically drives a pressure anisotropy with p ,j 1 p^,j (where p^,j and p ,j correspond to the pressure of species j perpendicular and parallel to B). In this work we studied the nonlinear, saturated properties of these instabilities, making use of particle-in-cell (PIC) simulations This is achieved by continuously decreasing the strength of the background magnetic field by externally imposing a shear motion in the plasma. Previous works have already studied this long-term regime by simulating an expanding (instead of shearing) plasma These works have used both hybrid-PIC simulations, which focused on the evolution of the ion anisotropy-driven instabilities (Matteini et al 2006; Hellinger & Travnicek 2008), and PIC simulations that mainly captured the role of electron anisotropy-driven modes (Camporeale & Burgess 2010). Our work is intended to study the combined effect of the electron and ion pressure anisotropies on the nonlinear, saturated regime of the different unstable modes.
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