Collisionless Rayleigh-Taylor-like instability of the boundary between a hot pair plasma and an electron-proton plasma: the undular mode

TL;DR

This study uses 2D particle-in-cell simulations to investigate a Rayleigh-Taylor-like instability at the boundary between a hot pair plasma and an electron-proton plasma, revealing how magnetic fields and instabilities influence plasma dynamics in astrophysical jets.

Contribution

It demonstrates the collisionless undular mode of Rayleigh-Taylor-like instability at the boundary between pair and electron-proton plasmas, highlighting magnetic field effects and piston deformation.

Findings

The expanding pair cloud expels and amplifies magnetic fields.

A solitary wave develops into a piston that undergoes undular instability.

The instability deforms but does not destroy the piston.

Abstract

We study with a two-dimensional particle-in-cell simulation the stability of a discontinuity or piston, which separates an electron-positron cloud from a cooler electron-proton plasma. Such a piston might be present in the relativistic jets of accreting black holes separating the jet material from the surrounding ambient plasma and when pair clouds form during an X-ray flare and expand into the plasma of the accretion disk corona. We inject a pair plasma at a simulation boundary with a mildly relativistic temperature and mean speed. It flows across a spatially uniform electron-proton plasma, which is permeated by a background magnetic field. The magnetic field is aligned with one simulation direction and oriented orthogonally to the mean velocity vector of the pair cloud. The expanding pair cloud expels the magnetic field and piles it up at its front. It is amplified to a value large enough to trap ambient electrons. The current of the trapped electrons, which are carried with the expanding cloud front, drives an electric field that accelerates protons. A solitary wave grows and changes into a piston after it saturated. Our simulations show that this piston undergoes a collision-less instability similar to a Rayleigh-Taylor instability. The undular mode grows and we observe fingers in the proton density distribution. The effect of the instability is to deform the piston but it cannot destroy it.