Field line diffusion in solar wind magnetic turbulence and energetic particle propagation across heliographic latitudes

The transport of energetic particles in the heliosphere is strongly influenced by the magnetohydrodynamic turbulence found in the solar wind. This turbulence causes a magnetic field line random walk, which can explain the Ulysses observations at high heliographic latitudes of particles accelerated at corotating interaction regions (CIRs). A three-dimensional model of magnetic turbulence allows us to evaluate a nonquasi-linear magnetic field line diffusion coefficient, even in the case of anisotropic turbulence, as observed in the solar wind. The excursion in latitude of a magnetic field line starting from a CIR located at a large heliospheric distance r(0) similar to 8-10 AU, and at the limiting latitude for the direct observation of CIRs, I similar to 35 degrees, is evaluated with a Monte Carlo simulation. In the calculations the random "force" terms are proportional to the square root of the diffusion coefficient in each direction. Considering that the correlation length in the latitudinal direction is larger than the one in the direction perpendicular to the mean magnetic field, we find that the magnetic field lines travel faster in the latitudinal direction. The implications for energetic particle transport are discussed and compared to the observation of energetic particle events at high solar latitudes by the Ulysses spacecraft. Both ion observations and electron observations can be explained by field line random walk.