Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks

Abstract

We present results of two-and three-dimensional Particle-In-Cell simulations of magnetic-turbulence production by isotropic cosmic-ray ions drifting upstream of supernova remnant shocks. The studies aim at testing recent predictions of a strong amplification of short-wavelength magnetic field and at studying the subsequent evolution of the magnetic turbulence and its backreaction on cosmic ray trajectories. We observe that an oblique filamentary mode grows more rapidly than the non-resonant parallel modes found in analytical theory, and the growth rate of the field perturbations is much slower than is estimated for the parallel plane-wave mode, possibly because in our simulations we cannot maintain ω ≪ Ω i , the ion gyrofrequency, to the degree required for the plane-wave mode to emerge. The evolved oblique filamentary mode was also observed in MHD simulations to dominate in the non-linear phase, when the structures are already isotropic. We thus confirm the generation of the turbulent magnetic field due to the drift of cosmic-ray ions in the upstream plasma, but as our main result find that the amplitude of the turbulence saturates at about δB/B ∼ 1. The backreaction of the magnetic turbulence on the particles leads to an alignment of the bulk-flow velocities of the cosmic rays and the background medium. This is an essential characteristic of cosmic-ray modified shocks: the upstream flow speed is continuously changed by the cosmic rays. The deceleration of the cosmic-ray drift

Multidimensional electron beam-plasma instabilities in the relativistic regime

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The microphysics and macrophysics of cosmic rays

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