Cosmic Ray Pitch Angle Scattering in Isotropic Turbulence
J. W. Bieber, C. W. Smith and W. H. Matthaeus
The Astrophysical Journal, 334, 470-475 (1988)
The divergent conclusions emerging from previous investigations of cosmic-ray pitch-angle scattering in isotropic magnetic turbulence can be reconciled by incorporating a dissipation range in the turbulence model. Such a dissipation range, where the omnidirectional energy spectrum falls off faster than k(exp-3) (k is the magnitude of the wavevector), can be expected to occur at scales smaller than some finite value, which we denote the ``dissipation scale.'' Utilizing an isotropic turbulence model that includes dissipation, we calculate the Fokker-Planck coefficient for pitch-angle scattering and find that the ``slab'' form of the Fokker-Planck coefficient favored by Jokipii is valid at very low energies. A smooth, gradual transition between the two forms occurs at energies such that the particle Larmor radius is of the order of the dissipation scale.
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