Enhancement of the non-resonant streaming instability by particle collisions

TL;DR

This paper investigates how particle collisions influence the non-resonant streaming instability driven by cosmic rays, revealing that Coulomb collisions can enhance magnetic field amplification in fully ionized plasmas.

Contribution

It presents hybrid-Particle-In-Cell simulations including Monte Carlo collisions to explore the interplay between pressure anisotropies and particle collisions in the instability.

Findings

Proton-neutral collisions damp the instability in poorly ionized plasmas.

Coulomb collisions in fully ionized plasmas can suppress pressure anisotropies.

Under certain conditions, collisions enhance magnetic field growth.

Abstract

Streaming cosmic rays can power the exponential growth of a seed magnetic field by exciting a non-resonant instability that feeds on their bulk kinetic energy. By generating the necessary turbulent magnetic field, it is thought to play a key role in the confinement and acceleration of cosmic rays at shocks. In this work we present hybrid-Particle-In-Cell simulations of the non-resonant mode including Monte Carlo collisions, and investigate the interplay between the pressure anisotropies produced by the instability and particle collisions in the background plasma. Simulations of poorly ionized plasmas confirm the rapid damping of the instability by proton-neutral collisions predicted by linear fluid theory calculations. In contrast we find that Coulomb collisions in fully ionized plasmas do not oppose the growth of the magnetic field, but under certain conditions suppress the pressure anisotropies and actually enhance the magnetic field amplification.