Cosmic Ray transport through gyroresonance instability in compressible turbulence

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).