Quasi‐linear Theory of Cosmic Ray Transport and Acceleration: The Role of Oblique Magnetohydrodynamic Waves and Transit‐Time Damping

Abstract

We calculate quasi-linear transport and acceleration parameters for cosmic ray particles interacting resonantly with undamped fast-mode waves propagating in a low-β plasma. For super-Alfvénic particles and a vanishing cross-helicity state of the fast-mode waves, we demonstrate that the rate of adiabatic deceleration vanishes, and that the momentum and spatial diffusion coefficients can be calculated from the Fokker-Planck coefficient D μμ . Adopting isotropic fast-mode turbulence with a Kolmogorov-like turbulence spectrum, we demonstrate that D μμ is the sum of contributions from transit-time damping and gyroresonant interactions. Gyroresonance refers to | n | ≠ 0 resonant particle-wave interactions. Transit-time damping refers to the n = 0 interaction of particles with the compressive magnetic field component of the fast-mode waves. We show that transit-time damping provides the dominant contribution to pitch-angle scattering in the interval ##IMG## [http://ej.iop.org/icons/Entities/epsi.gif] {epsilon} ≤ | μ | ≤ 1, where ##IMG## [http://ej.iop.org/icons/Entities/epsi.gif] {epsilon} is the ratio of Alfvén to particle speed. In the interval | μ |