The Ratio of Perpendicular and Parallel Diffusion Coefficients of Low-energy Particles in Turbulent Space Plasmas

Recently an improved nonlinear theory for the transport of energetic particles across a mean magnetic field has been developed. The latter theory is called the field line-particle decorrelation theory and is the first analytical theory that agrees with test-particle simulations without the need of a correction parameter, nor does the theory contain any other free parameter. In the current paper we derive analytical forms for the ratio of perpendicular and parallel spatial diffusion coefficients kappa(perpendicular to)/kappa(parallel to )of low-energy particles. In the considered limit the latter ratio is constant meaning that it does not depend on particle energy or rigidity. It is shown that the ratio always has the form kappa(perpendicular to)/kappa(parallel to ) = a(2)delta B-x(2)/B-0(2) if a two-dimensional turbulence model is employed. Furthermore, the parameter a(2) depends only on the shape of the turbulence spectrum but not on the magnetic fields. The obtained results can be important for a variety of applications such as studies of solar modulation and diffusive shock acceleration.