PIC simulation of a thermal anisotropy-driven Weibel instability in a circular rarefaction wave

TL;DR

This study uses PIC simulations to demonstrate that a thermal anisotropy-driven Weibel instability can develop in a curved rarefaction wave, generating magnetic fields relevant to astrophysical phenomena.

Contribution

It extends previous work by showing TAWI growth in a circular plasma expansion, revealing new magnetic instability mechanisms in curved geometries.

Findings

Radial current channels form during plasma expansion.

Aperiodic growth of a magnetowave with orthogonal magnetic fields is observed.

A secondary magnetic instability leads to a rotational low-frequency magnetowave.

Abstract

The expansion of an initially unmagnetized planar rarefaction wave has recently been shown to trigger a thermal anisotropy-driven Weibel instability (TAWI), which can generate magnetic fields from noise levels. It is examined here if the TAWI can also grow in a curved rarefaction wave. The expansion of an initially unmagnetized circular plasma cloud, which consists of protons and hot electrons, into a vacuum is modelled for this purpose with a two-dimensional particle-in-cell (PIC) simulation. It is shown that the momentum transfer from the electrons to the radially accelerating protons can indeed trigger a TAWI. Radial current channels form and the aperiodic growth of a magnetowave is observed, which has a magnetic field that is oriented orthogonal to the simulation plane. The induced electric field implies that the electron density gradient is no longer parallel to the electric field. Evidence is presented here for that this electric field modification triggers a second magnetic instability, which results in a rotational low-frequency magnetowave. The relevance of the TAWI is discussed for the growth of small-scale magnetic fields in astrophysical environments, which are needed to explain the electromagnetic emissions by astrophysical jets. It is outlined how this instability could be examined experimentally.