Magnetic turbulence production by streaming cosmic rays upstream of SNR shocks

TL;DR

This paper uses Particle-In-Cell simulations to study magnetic turbulence caused by cosmic rays upstream of supernova remnant shocks, testing MHD predictions and exploring turbulence evolution and backreaction effects.

Contribution

It provides the first kinetic simulation results confirming magnetic turbulence generation and evaluates the growth rate and saturation level, challenging previous MHD estimates.

Findings

Magnetic turbulence is generated by cosmic-ray streaming.

Growth rate of magnetic perturbations is slower than MHD predictions.

Turbulence saturates at about 10% of the background magnetic field.

Abstract

We present preliminary results of Particle-In-Cell simulations of magnetic turbulence production by isotropic cosmic-ray ions streaming upstream of supernova remnant shocks. The studies aim at testing the MHD predictions by Bell (2004, 2005) of a strong amplification of short-wavelength nonresonant wave modes and at studying the subsequent evolution of the magnetic turbulence and its backreaction on cosmic ray trajectories. The detailed knowledge of the upstream turbulence properties is crucial to ascertain all aspects of the shock acceleration process - the transport properties of cosmic rays, the shock structure, thermal particle injection and heating processes. An amplification of magnetic field would also facilitate the acceleration of particles beyond the "knee" in the cosmic-ray spectrum. Our kinetic approach is particularly suited to address the backreaction on the cosmic rays, and it allows us to test Bell's prediction of the eventual formation of extended filamentary structure in the cosmic-ray distribution and also to arrive at a reliable estimate of the total saturation magnetic-field level. The parameters chosen for the simulations are favorable for the rapid excitation of purely growing modes. We confirm the generation of the turbulent magnetic field due to the drift of cosmic-ray ions in the upstream plasma, but show that the growth rate of the field perturbations is much slower than estimated using the MHD approach and the amplitude of the turbulence saturates at about dB/B~1. The magnetic field also remains below equipartition with the upstream plasma.