QUASI-LINEAR THEORY OF COSMIC-RAY TRANSPORT AND ACCELERATION - THE ROLE OF OBLIQUE MAGNETOHYDRODYNAMIC WAVES AND TRANSIT-TIME DAMPING

We calculate quasi-linear transport and acceleration parameters for co smic ray particles interacting resonantly with undamped fast-mode wave s propagating in a low-beta plasma. For super-Alfvenic particles and a vanishing cross-helicity state of the fast-mode waves, we demonstrate that the rate of adiabatic deceleration vanishes, and that the moment um and spatial diffusion coefficients can be calculated from the Fokke r-Planck coefficient D-mu mu. Adopting isotropic fast-mode turbulence with a Kolmogorov-like turbulence spectrum, we demonstrate that D-mu m u is the sum of contributions from transit-time damping and gyroresona nt interactions. Gyroresonance refers to \n\not equal 0 resonant parti cle-wave interactions. Transit-time damping refers to the n=0 interact ion of particles with the compressive magnetic field component of the fast-mode waves. We show that transit-time damping provides the domina nt contribution to pitch-angle scattering in the interval epsilon less than or equal to\mu\less than or equal to 1, where epsilon is the rat io of Alfven to particle speed. In the interval \mu\<epsilon, transit- time damping does not occur, and gyroresonance provides a small but fi nite contribution to particle scattering. As a consequence, the moment um diffusion coefficient is mainly determined by the transit-time damp ing contribution. On the other hand, since the spatial diffusion coeff icient and the related mean free path are given by the average over mu of the inverse of D-mu mu), these spatial transport parameters are de termined by the contribution from the interval \mu\<epsilon. Pie also calculate the cosmic ray transport parameters for plasma turbulence co nsisting of a mixture of isotropic fast-mode waves and slab Alfven wav es. Here, the momentum diffusion coefficient is determined by the tran sit-time damping of the fast-mode waves, and is a factor 1n epsilon(-1 ) larger than in the case of pure slab Alfven wave turbulence. The mea n free path and the spatial diffusion coefficient are modified signifi cantly from the pure fast-mode case, since the crucial scattering at \ mu\<epsilon is now provided by gyroresonances with slab Alfven waves. The mean free path is a constant at nonrelativistic energies, and may account for the legendary lambda(fit)-lambda(QLT) discrepancy of sella r energetic particles.