Cosmic Ray transport through gyroresonance instability in compressible turbulence

TL;DR

This paper investigates how cosmic rays interact with and are scattered by gyroresonance instabilities driven by large-scale compressible turbulence, revealing a steady state balance and the dominant scattering mechanisms for high-energy cosmic rays.

Contribution

It introduces a nonlinear model of cosmic ray scattering via gyroresonance instability in compressible turbulence, highlighting the saturation effects and energy range limitations.

Findings

Nonlinear wave-particle interactions saturate the instability growth.

High-energy cosmic rays (> 100 GeV) are mainly isotropized by large-scale compressible modes.

Energy range for cosmic ray scattering is limited by wave-particle scattering.

Abstract

We study the nonlinear growth of kinetic gyroresonance instability of cosmic rays (CRs) induced by large scale compressible turbulence. This feedback of cosmic rays on turbulence was shown to induce an important scattering mechanism in addition to direct interaction with the compressible turbulence. The linear growth is bound to saturate due to the wave-particle interactions. By balancing increase of CR anisotropy via the large scale compression and its decrease via the wave-particle scattering, we find the steady state solutions. The nonlinear suppression due to the wave-particle scattering limit the energy range of CRs that can excite the instabilities and be scattered by the induced slab waves. The direct interaction with large scale compressible modes still appears to be the dominant mechanism for isotropization of high energy cosmic rays (> 100 GeV).