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

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

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...492..352s&link_type=abstract

Astrophysical Journal v.492, p.352

Astronomy and Astrophysics

Astronomy

85

Acceleration Of Particles, Ism: Cosmic Rays, Diffusion, Plasmas, Sun: Particle Emission

Scientific paper

We calculate quasi-linear transport and acceleration parameters for cosmic ray particles interacting resonantly with undamped fast-mode waves 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 momentum and spatial diffusion coefficients can be calculated from the Fokker-Planck coefficient D mu mu . Adopting isotropic fast-mode turbulence with a Kolmogorov-like turbulence spectrum, we demonstrate that D mu mu 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 epsilon <= | mu | <= 1, where epsilon is the ratio of Alfven to particle speed. In the interval | mu | < epsilon , transit-time damping does not occur, and gyroresonance provides a small but finite contribution to particle scattering. As a consequence, the momentum diffusion coefficient is mainly determined by the transit-time damping contribution. On the other hand, since the spatial diffusion coefficient 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 determined by the contribution from the interval | mu | < epsilon . We also calculate the cosmic ray transport parameters for plasma turbulence consisting of a mixture of isotropic fast-mode waves and slab Alfven waves. Here, the momentum diffusion coefficient is determined by the transit-time damping of the fast-mode waves, and is a factor ln epsilon -1 larger than in the case of pure slab Alfven wave turbulence. The mean free path and the spatial diffusion coefficient are modified significantly 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 solar energetic particles.

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