Scattering of cosmic rays by magnetohydrodynamic interstellar turbulence.

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in our understanding of cosmic ray transport. We use recently obtained scalings of MHD modes to calculate the scattering frequency for cosmic rays. We consider gyroresonance with MHD modes (Alfvénic, slow, and fast) and transit-time damping by fast modes. We conclude that the gyroresonance with fast modes is the dominant contribution to cosmic ray scattering for the typical interstellar conditions. In contrast to earlier studies, we find that Alfvénic and slow modes are inefficient because they are far from the isotropy usually assumed.