Cosmic‐Ray Scattering and Streaming in Compressible Magnetohydrodynamic Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence callfor revisions in the picture of cosmic ray transport. In this paper we userecently obtained scaling laws for MHD modes to obtain the scattering frequencyfor cosmic rays. Using quasilinear theory we calculate gyroresonance with MHDmodes (Alfv\'{e}nic, slow and fast) and transit-time damping (TTD) by fastmodes. We provide calculations of cosmic ray scattering for various phases ofinterstellar medium with realistic interstellar turbulence driving that isconsistent with the velocity dispersions observed in diffuse gas. We accountfor the turbulence cutoff arising from both collisional and collisionlessdamping. We obtain analytical expressions for diffusion coefficients that enterFokker-Planck equation describing cosmic ray evolution. We obtain thescattering rate and show that fast modes provide the dominant contribution tocosmic ray scattering for the typical interstellar conditions in spite of thefact that fast modes are subjected to damping. We determine how the efficiencyof the scattering depends on the characteristics of ionized media, e.g. plasma$\beta$. We calculate the range of energies for which the streaming instabilityis suppressed by the ambient MHD turbulence.Comment: 13 pages, 5 figures, to appear in ApJ 20 October 2004, v61