We studied a magnetic turbulence axisymmetric around the unperturbed magnetic field for cases having different ratios l( ||)/l( perpendicular). We find, in addition to the fact that a higher fluctuation level deltaB/B(0) makes the system more stochastic, that by increasing the ratio l( ||)/l( perpendicular) at fixed deltaB/B(0), the stochasticity increases. It appears that the different transport regimes can be organized in terms of the Kubo number R=(deltaB/B(0))(l( ||)/l( perpendicular)). The simulation results are compared with the two analytical limits, that is the percolative limit and the quasilinear limit. When R<<1 weak chaos, closed magnetic surfaces, and anomalous transport regimes are found. When R approximately 1 the diffusion regime is Gaussian, and the quasilinear scaling of the diffusion coefficient D( perpendicular) approximately (deltaB/B(0))(2) is recovered. Finally, for R>>1 the percolation scaling of the diffusion coefficient D( perpendicular) approximately (deltaB/B(0))(0.7) is obtained.

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The transport of energetic particles in a mean magnetic field and in the presence of anisotropic magnetic turbulence is studied numerically, for parameter values relevant to astrophysical plasmas. A numerical realization of magnetic turbulence is set up, in which the degree of anisotropy is varied by changing the correlation lengths l x , l y , and l z . The ratio / of the particle Larmor radius over the turbulence correlation length is also varied. It is found that for l x ,l y ӷ l z , and for / Շ10 −2 transport can be non-Gaussian, with superdiffusion along the average magnetic field and subdiffusion perpendicular to it. In addition, the spatial distribution of particles is clearly non-Gaussian. Such regimes are characterized by a Levy statistics, with diverging second-order moments. Decreasing the ratio l x / l z , nearly Gaussian ͑normal͒ diffusion is obtained, showing that the transport regime depends on the turbulence anisotropy. Changing the particle Larmor radius, normal diffusion is found for 10 −2 Շ / Շ1 because of increased pitch angle diffusion. New anomalous superdiffusive regimes appear when / տ1 showing that the interaction between particles and turbulence decreases in these cases. A new regime, called generalized double diffusion, is proposed for the cases when particles are able to trace back field lines. A summary of the physical conditions which lead to non-Gaussian transport is given.

The magnetic turbulence in the solar wind causes a magnetic field line transport that is reflected in the propagation in space of charged particles. Assuming a small localized source, the distribution in space of energetic particles is determined, in part, by the shape of the magnetic flux tube. The spatial evolution of a magnetic flux tube is studied here by means of a numerical realization of three‐dimensional magnetic turbulence that takes into account the anisotropy of the solar wind turbulence and is quantified by correlation lengths in the three spatial directions. Several diagnostics of flux tube evolution are shown, such as patterns of the flux tube cross sections and histograms representing possible energetic particle intensity profiles. We show that flux tube evolution can be assessed by the Kubo number R = (δB/B0)(lz/lx), where δB/B0 is the turbulence level and lz (lx) is the correlation length parallel (transversal) to the background magnetic field B0. We find that when lz/lx (i.e., R) is large, the flux tube evolves very quickly, forming very fine, diffusive structures. These diffusive structures would correspond to a nearly Gaussian envelope for the energetic particle time profile detected by a spacecraft. On the other hand, when lz/lx is small, the flux tube evolves slowly, executing large coherent transverse displacements, and thereby forms well resolved (i.e., detached) intermittent high particle fluxes in observed energetic particle profiles. Hence an accurate study of the morphology of impulsive energetic particle events, when compared with our simulation results, can give information on the microphysical evolution of flux tubes in the solar wind and on the turbulence anisotropy. A first comparison indicates that lx ≃ 3–10lz is appropriate for the solar wind. Our study also allows us to reconcile fast field line transport with the observation of sharp composition or energetic particle gradients in the solar wind, since lx ≫ lz implies considerable transverse elongation of the flux tube cross section, with the possibility of non‐Gaussian, superdiffusive transport regimes.