PIC Simulations of Velocity-space Instabilities in a Decreasing Magnetic Field: Viscosity and Thermal Conduction

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.