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 influen ced by the magnetohydrodynamic turbulence found in the solar wind. This tur bulence causes a magnetic field line random walk, which can explain the Uly sses observations at high heliographic latitudes of particles accelerated a t corotating interaction regions (CIRs). A three-dimensional model of magne tic turbulence allows us to evaluate a nonquasi-linear magnetic field line diffusion coefficient, even in the case of anisotropic turbulence, as obser ved in the solar wind. The excursion in latitude of a magnetic field line s tarting 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 correl ation length in the latitudinal direction is larger than the one in the dir ection perpendicular to the mean magnetic field, we find that the magnetic field lines travel faster in the latitudinal direction. The implications fo r energetic particle transport are discussed and compared to the observatio n of energetic particle events at high solar latitudes by the Ulysses space craft. Both ion observations and electron observations can be explained by field line random walk.