Simulation studies of temperature anisotropy driven pair-Alfvén and aperiodic instabilities in magnetized pair plasma

M E Dieckmann,R Walder,A Bret,D Folini

doi:10.1088/1361-6587/ab2b2d

Abstract

We compare with one-dimensional particle-in-cell simulations the aperiodically growing instabilities driven by a bi-Maxwellian velocity distribution in unmagnetized electron plasma (Weibel instability) and in pair plasma. The simulation box is aligned with the cool direction. The waves in both simulations evolve towards a circularly polarized non-propagating magnetic structure. Its current and magnetic field are aligned and the structure is in a force-free state. We examine how a background magnetic field B0, which is parallel to the simulation direction, affects the waves in the pair plasma. A weak B0 cannot inhibit the growth of the aperiodically growing instability but it prevents it from reaching the force-free stable state. The mode collapses and seeds a pair Alfvén waves. An intermediate B0 couples the thermal anisotropy to the pair Alfvén mode and propagating magnetowaves grow. The phase speed of the pair of Alfvén waves is increased by the thermal anisotropy. Its growth is suppressed when B0 is set to the value that stabilizes the mirror mode.

Highlights

Plasma far from a thermal equilibrium, in which binary collisions between particles are infrequent on the time scales of interest, relaxes via the growth of electromagnetic waves
We compare with one-dimensional particle-in-cell simulations the aperiodically growing instabilities driven by a bi-Maxwellian velocity distribution in unmagnetized electron plasma (Weibel instability) and in pair plasma
The cool direction and the magnetic field were parallel to the simulation direction

Summary

Introduction

Plasma far from a thermal equilibrium, in which binary collisions between particles are infrequent on the time scales of interest, relaxes via the growth of electromagnetic waves. The aforementioned instabilities may develop in the unstable inner accretion discs of black hole binaries that reached a temperature that is high enough to generate electron-positron pairs [23], in pair jets that flow along a guiding magnetic field [24] and in the transition layers of their internal collisionless shocks [25, 26] These instabilities can dissipate the energy of the inflowing upstream material in the pair shocks that are thought to exist in energetic astrophysical environments [27]. We show by means of simulations and for our initial conditions that the value of B0 that suppresses the mirror instability stabilizes the aperiodically growing mode and the pair Alfvén mode.

The code

Initial conditions

Instabilities in unmagnetized electron plasma and pair plasma

Magnetized pair plasmas

Findings

Summary