Drift and Diffusion of Energetic Charged Particles in Nonaxisymmetric Two-Component Magnetic Turbulence with Nonuniform Large Scale Field

The guiding center drift in a nonuniform magnetic field is an important process in energetic particle transport and solar modulation of cosmic rays. A turbulent magnetic field is known to reduce the particle drift, but studies to date have mainly focused on axisymmetric turbulence, whereas solar wind expansion is expected to stretch turbulent structures preferentially in certain directions. In this study, we discuss both theory and numerical simulations about the perpendicular transport of energetic charged particles in nonaxisymmetric two-component (2D+slab) turbulence with a nonuniform background field in terms of the Fokker-Planck coefficients of drift and diffusion in each direction. We numerically trace trajectories of charged particles in this model magnetic field, where the large scale field, in the z-direction, is a function of x and we vary an ellipticity parameter ξ that controls the nonaxisymmetry of the 2D+slab turbulence. The 2D turbulence is made nonaxisymmetric by stretching the 2D structures along x or y. Then we compute the ensemble average drift and diffusion coefficients to explore the particle transport in various cases, in comparison with theoretical results. This work was partially supported by the Thailand Research Fund award RTA5980003.