Cosmic Ray Scattering by Compressible Magnetohydrodynamic Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in the picture of cosmic ray transport. In this paper we use recently obtained scaling laws for MHD modes to calculate the scattering frequency for cosmic rays in the ISM. We consider gyroresonance with MHD modes (Alfvénic, slow and fast) and transit-time damping (TTD) by fast modes. We provide calculations of cosmic ray scattering for various phases of interstellar medium with realistic interstellar turbulence driving that is consistent with the velocity dispersions observed in diffuse gas. We account for the turbulence cutoff arising from both collisional and collisionless damping. We obtain analytical expressions for diffusion coefficients that enter Fokker-Planck equation describing cosmic ray evolution. We calculate the scattering rate and parallel spatial diffusion coefficients of cosmic rays for both Alfvénic and fast modes. We conclude that fast modes provides the dominant contribution to cosmic ray scattering for the typical interstellar conditions in spite of the fact that fast modes are subjected to damping. We show that the efficiency of the scattering depends on the plasma β since it determines the damping of the fast modes. We also show that the streaming instability is modified in the presence of turbulence. Subject headings: acceleration of particles–cosmic rays–ISM: magnetic fields– MHD–scattering–turbulence