Cosmic Ray transport in turbulent magnetic field

TL;DR

This paper investigates how magnetic turbulence affects cosmic ray transport and acceleration, emphasizing the roles of compressible fast modes and gyroresonance instability in scattering processes.

Contribution

It introduces a comprehensive analysis of cosmic ray scattering mechanisms in turbulent magnetic fields, highlighting the importance of compressible modes and nonlinear effects.

Findings

Fast modes dominate cosmic ray scattering.

Self-generated waves are constrained by existing turbulence.

Gyroresonance instability significantly impacts scattering.

Abstract

Cosmic ray (CR) transport and acceleration is determined by the properties of magnetic turbulence. Recent advances in MHD turbulence call for revisions in the paradigm of cosmic ray transport. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulation, in which turbulence is injected at large scale and cascades to to small scales. We shall address the issue of the transport of CRs, both parallel and perpendicular to the magnetic field. We shall demonstrate compressible fast modes are dominant cosmic ray scatterer from both quasilinear and nonlinear theories. We shall also show that the self-generated wave growth by CRs are constrained by preexisting turbulence and discuss the process in detail in the context of shock acceleration at supernova remnants and their implications. In addition, we shall dwell on the nonlinear growth of kinetic gyroresonance instability of cosmic rays induced by large scale compressible turbulence. This gyroresonance of cosmic rays on turbulence is demonstrated an important scattering mechanism in addition to direct interaction with the compressible turbulence. The feedback of the instability on large scale turbulence should be included in future simulations.