Kinetic simulations of turbulent magnetic-field growth by streaming cosmic rays

TL;DR

This paper uses multidimensional particle-in-cell simulations to study how streaming cosmic rays amplify magnetic fields upstream of shocks, revealing saturation, turbulence, and modifications to cosmic-ray and shock properties.

Contribution

It provides the first detailed simulation-based analysis of magnetic field amplification by cosmic-ray streaming and its effects on the upstream medium and shock structure.

Findings

Magnetic field amplification saturates similarly across parameters.

Cosmic-ray back-reaction modifies their distribution and momentum transfer.

Upstream turbulence and fluctuations influence shock properties.

Abstract

Efficient acceleration of cosmic rays (via the mechanism of diffusive shock acceleration) requires turbulent, amplified magnetic fields in the shock's upstream region. We present results of multidimensional particle-in-cell simulations aimed at observing the magnetic field amplification that is expected to arise from the cosmic-ray current ahead of the shock, and the impact on the properties of the upstream interstellar medium. We find that the initial structure and peak strength of the amplified field is somewhat sensitive to the choice of parameters, but that the field growth saturates in a similar manner in all cases: the back-reaction on the cosmic rays leads to modification of their rest-frame distribution and also a net transfer of momentum to the interstellar medium, substantially weakening their relative drift while also implying the development of a modified shock. The upstream medium becomes turbulent, with significant spatial fluctuations in density and velocity, the latter in particular leading to moderate upstream heating; such fluctuations will also have a strong influence on the shock structure.