Acoustic instability at shock-wave precursors

TL;DR

This study uses magnetohydrodynamical simulations to explore how acoustic instabilities amplify magnetic fields in shock precursors, impacting cosmic-ray acceleration in supernova remnants.

Contribution

It demonstrates that acoustic instability can generate large nonlinear structures from small density perturbations at cosmic-ray-modified shocks using realistic parameters.

Findings

Acoustic instability transforms small perturbations into large nonlinear structures.

Turbulent magnetic fluctuations are characterized by a specific power spectrum.

Constructive interference between instabilities may enhance magnetic field amplification.

Abstract

Magnetic field amplification is an integral part of the process of particle acceleration at non-relativistic shocks. It is necessary to reach the maximum energies required by observations, especially in supernova remnants, thought to be sources of the bulk of Galactic cosmic rays. Such amplification can be caused by the acoustic instability that develops when small density perturbations interact with the cosmic-ray pressure gradient in the upstream of a cosmic-ray-modified shock. The vorticity induced by the nonlinear development of the instability may lead to turbulence, which amplifies the pre-existing magnetic fields. To study this phenomenon, we use the PLUTO code to carry out 2D (and some 3D) magnetohydrodynamical simulations of the evolution of small density perturbations in the presence of an assigned cosmic-ray pressure gradient. Adopting more realistic values of Mach number and cosmic-ray acceleration efficiency than previously assumed in the literature, we show that the acoustic instability can transform small density perturbations into large nonlinear structures while the fluid crosses the precursor region of a cosmic-ray-modified shock. We study the power spectrum of turbulent magnetic fluctuations that may be important to scatter particles. We comment on the possible constructive interference between acoustic and non-resonant streaming instabilities. We discuss limitations of previous and current numerical investigations in accessing spatial scales where turbulence is expected to turn nonlinear, and outline perspectives for future investigations.