Parallel heat diffusion and subdiffusion in random magnetic fields

Stochastic magnetic fields in a weakly collisional plasma enhance the collisionality of electrons in the medium compared to the classical Spitzer values. The reduction in the thermal conductivity may play an important role in the evolution of galaxy clusters. This work extends prior work by examining the parallel propagation of electrons in plasma media where the magnetic field amplitude sampled along a field line is a stochastic function of the distance. New physics is exhibited when many large-amplitude magnetic mirrors are present; the parallel transport becomes subdiffusive rather than diffusive in this limit. The transition between diffusion and subdiffusion can be obtained from properties of the asymptotic solution to the kinetic equation in the vicinity of a solitary magnetic mirror. This transition has been computed as a function of the power law exponent in the distribution of mirror maxima, and the asymptotic scaling of the mean-squared electron displacement with time has been obtained in these systems as a function of this exponent. Monte Carlo comparisons are in agreement with these results. (C) 2001 American Institute of Physics.