A perturbation approach to coherent propagation of energetic charged particles in random magnetic fields

The coherent propagation of energetic charged particles in random magnetic fields first discussed by Earl is extended to more general magnetic field models by using a perturbation approach. Furthermore, it is shown that the parameter range for the slab model, for which coherent propagation holds, is larger than found by Earl. In the overlapping parameter region the transport coefficients given by Earl differ from the ones calculated in this paper, although they are of the same order of magnitude if the spectral index q is not too close to q=-3. For isotropic magnetic field fluctuations no pure diffusive solutions of the Fokker-Planck equation exist. However, coherent propagation is possible for spectral indices q between -3 and -1