Two-Dimensional particle-in-cell simulations of the nonresonant, cosmic-ray driven instability in SNR shocks

TL;DR

This paper uses two-dimensional particle-in-cell simulations to study the nonresonant cosmic-ray driven instability in supernova remnant shocks, revealing magnetic field amplification and saturation mechanisms.

Contribution

It provides the first detailed simulation of the instability, confirming theoretical predictions and analyzing its saturation behavior in a controlled environment.

Findings

Magnetic energy density can grow at least 10 times initial value

Circularly polarized magnetic waves are observed as predicted

Saturation occurs due to boundary effects, not physical mechanisms

Abstract

In supernova remnants, the nonlinear amplification of magnetic fields upstream of collisionless shocks is essential for the acceleration of cosmic rays to the energy of the "knee" at 10^{15.5}eV. A nonresonant instability driven by the cosmic ray current is thought to be responsible for this effect. We perform two-dimensional, particle-in-cell simulations of this instability. We observe an initial growth of circularly polarized non-propagating magnetic waves as predicted in linear theory. It is demonstrated that in some cases the magnetic energy density in the growing waves, can grow to at least 10 times its initial value. We find no evidence of competing modes, nor of significant modification by thermal effects. At late times we observe saturation of the instability in the simulation, but the mechanism responsible is an artefact of the periodic boundary conditions and has no counterpart in the supernova-shock scenario.