Separation of Charged Particles from Magnetic Field Lines in Two-Component Magnetic Turbulence

In interplanetary space, the transport of energetic charged particles is influenced by a turbulent magnetic field. Previous studies have shown that a two-component (2D+slab) magnetic model of turbulence is a useful model for the magnetic field in the heliosphere. Normally, the diffusive behavior of charged particles in a turbulent magnetic field is observed when they approach the long time limit. The charged particles are often assumed to follow and diffuse according to the random walk of the field lines but some theories of perpendicular particle transport, such as nonlinear guiding center theory (NLGC), implicitly assume some true cross-field diffusion in which particles separate from the field line connected to their initial location. Furthermore, such cross-field diffusion is of specific interest because it is the only way that particles can diffuse across boundaries of magnetic field topology, such as the heliospheric current sheet and boundaries of interplanetary magnetic flux ropes. In this work, we study such cross-field diffusion using numerical techniques to simulate the trajectories of charged particles and magnetic field lines in two-component magnetic turbulence and to find the separation between the particles and their initial magnetic field lines. The guiding centers (GC) of the particles are computed here. Then we calculate the spreading between the GC of the particles and the trajectories of the field lines. We found that, in the pure slab turbulence, the particles stick with the magnetic field lines at which they initially start. In the 2D+slab case, the particles initially follow their initial field lines and then spread diffusively in the long time limit. We perform simulations for varying particle energy, ratio of 2D to slab fluctuations, and strength of the magnetic fluctuation in order to understand the relationship between the particle and magnetic field line trajectories. The diffusion coefficients of the particles have been calculated and compared with previous theories. This will lead to better understanding about the mechanisms of particle transport and will also help in developing a more complete transport theory of energetic charged particles in magnetic turbulence. Partially supported by the Thailand Research Fund, NSF SHINE ATM-0752135, and NASA Heliophysics Theory Program NNX08AI47G.