Parallel firehose instability in electron-positron plasmas

In a magnetized uniform plasma, firehose instability may arise as a result of pressure anisotropy of P>PâŠ1, where P and PâŠ1 are the thermal pressure parallel and perpendicular to the magnetic field, respectively. In this paper, we examine the parallel firehose instability in electron-positron plasmas based on the particle simulations along with the linear fluid theory, which may give rise to the dispersion relation, instability criteria, and growth rate, etc., for comparisons with those calculated from the kinetic simulations. As for the firehose instability in electron-proton plasmas, the magnetic field grows rapidly and then decreases with oscillations. The nonlinear saturated state complies with the linear stability criterion, α=μ0(P-PâŠ1)/B2=1, derived from the fluid theory only for relatively smaller values of initial α or ωp/ωc, where ωp and ωc are the plasma and cyclotron frequencies, respectively. For relatively larger values of initial α and ωp/ωc, the saturated α values are smaller than 1 as a result of kinetic resonant effects. The dominant wave numbers are kc/ωp<0.5 and the growth rates are in the range of 0.1-0.3ωc, which are approximately consistent with the linear fluid theory for the same wavelengths. Both electrostatic and electromagnetic modes predicted by the linear fluid theory are identified in the kinetic simulations.