Cosmic Ray transport in MHD turbulence: large and small scale interactions

TL;DR

This paper investigates cosmic ray transport in magnetohydrodynamic turbulence, revealing super-diffusive behavior at small scales and highlighting the significance of gyroresonance instability as a scattering mechanism, challenging previous paradigms.

Contribution

It introduces a revised understanding of cosmic ray transport, emphasizing super-diffusion at small scales and the role of gyroresonance instability in scattering processes.

Findings

CR transport becomes super-diffusive below turbulence injection scale

Gyroresonance instability significantly contributes to cosmic ray scattering

Feedback from instability affects large-scale turbulence dynamics

Abstract

Cosmic ray (CR) transport and acceleration is essential for many astrophysical problems, e.g., CMB foreground, ionization of molecular clouds and all high energy phenomena. 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 and show that the issue of cosmic ray subdiffusion is only important for restricted cases when the ambient turbulence is far from that suggested by numerical simulations. Moreover, on scales less than injection scale of turbulence, CRs's transport becomes super-diffusive. We also shall discuss 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 cannot be neglected, and should be included in future simulations.