Velocity Correlation and Decorrelation Time of Energetic Charged Particles in Non-axisymmetric Two-Component Magnetic Turbulence

Abstract:

The Taylor-Green-Kubo (TGK) equation is a well-known formula for studying the transport of charged particles. The velocity correlation function is the key that leads to both diagonal and off-diagonal diffusion coefficients (also called diffusion and drift coefficients). A previous study by Bieber and Matthaeus [1997] (BAM) expressed the velocity correlation function in terms of oscillatory exponential decay with time. In our work we present the relationship between the velocity correlation function and the diffusion and drift coefficients of charged particles in non-axisymmetric two-component (2D+slab) turbulence. The trajectories of particles are numerically traced and we directly compute the velocity correlation functions as well as the diffusion and drift coefficients. We found that, for low magnetic fluctuation levels, the mathematical form from BAM theory still provides a good fit to the simulation results even for non-axisymmetric cases. The decorrelation time changes only by a small factor and depends on the non-axisymmetric stretching of the turbulent structure, but not on the direction of stretch. For the strong magnetic fluctuation case, we suggest a new fitting function that is a combination of two oscillatory exponential decay terms. The adapted mathematical function can provide fitting parameters as decorrelation times and oscillation frequencies which lead us to better understand the perpendicular transport of charged particles in our heliosphere. This work was partially supported by grant RTA6280002 from Thailand Science Research and Innovation.