Two-dimensional particle simulation of the boundary between a hot pair plasma and magnetized electrons and protons: out-of-plane magnetic field

TL;DR

This study uses particle-in-cell simulations to explore how a relativistic pair plasma interacts with a magnetized electron-proton plasma, revealing the formation of a discontinuity layer relevant to astrophysical jets.

Contribution

It demonstrates the formation of a stable discontinuity layer between pair plasma and ambient plasma, highlighting electromagnetic effects and instabilities involved.

Findings

Pair plasma expels and piles magnetic field at its front.

Electromagnetic pulse separates pair plasma from ambient plasma.

Transition layer acts as a discontinuity, relevant for astrophysical jets.

Abstract

By means of a particle-in-cell (PIC) simulation, we study the interaction between a uniform magnetized ambient electron-proton plasma at rest and an unmagnetized pair plasma, which we inject at one simulation boundary with a mildly relativistic mean speed and temperature. The magnetic field points out of the simulation plane. The injected pair plasma expels the magnetic field and piles it up at its front. It traps ambient electrons and drags them across the protons. An electric field grows, which accelerates protons into the pair cloud's expansion direction. This electromagnetic pulse separates the pair cloud from the ambient plasma. Electrons and positrons, which drift in the pulse's nonuniform field, trigger an instability that disrupts the current sheet ahead of the pulse. The wave vector of the growing perturbation is orthogonal to the magnetic field direction and magnetic tension cannot stabilize it. The electromagnetic pulse becomes permeable for pair plasma, which forms new electromagnetic pulses ahead of the initial one. A transition layer develops with a thickness of a few proton skin depths, in which protons and positrons are accelerated by strong electromagnetic fields. Protons form dense clumps surrounded by a strong magnetic field. The thickness of the transition layer grows less rapidly than we would expect from the typical speeds of the pair plasma particles and the latter transfer momentum to protons; hence, the transition layer acts as a discontinuity, separating the pair plasma from the ambient plasma. Such a discontinuity is an important building block for astrophysical pair plasma jets.