Charged-Particle Transport in Turbulent Magnetic Fields

Abstract

Collisionless turbulence is common in astrophysical magnetic fields and plasmas. The determination of the transport of high-energy charged particles both parallel and perpendicular to the average magnetic field in such a system is of considerable interest. It is recognized that the turbulent magnetic field has important effects on the transport of charged particles and that the properties of different turbulence models may significantly affect the resulting transport properties. A number of different magnetic turbulence models have been proposed in the last several decades. We present here the results of a study of charged particle transport in two new turbulent magnetic field models that have not been previously considered and include newly described characteristics. We investigate the effect of energetic charged particle motion in these new models. We use a method (D A Roberts, 2012) that optimizes phase angles of a set of circularly polarized, transverse modes with Kolmogorov power-law enveloped amplitudes to construct magnetic field fluctuations with nearly constant |B| but with large variances in the components Bx, By, Bz, as is observed in the Solar Wind. Charged particle scattering coefficients are determined through computer simulations. The results are compared with those from previous isotropic and composite turbulence models. We studied charged particle transport in the turbulent magnetic field with global anisotropy and compared with the isotropic turbulence. We found that the magnetic turbulence with global anisotropy and isotropy have similar effects on charged particle transport from 1𝑀𝑒𝑉 up to 10𝐺𝑒𝑉. We proposed a general framework for a multi-scale synthesis with the scale- dependent, localized anisotropic feature incorporated. We run test particle simula- tions in the field by the two-scale algorithm to calculate the transport coefficients for charged particles with different energies. We found that the local anisotropy has the significant difference from the previous model in the effect on charged particle transport. The parallel transport (𝑘∥) decreases while the perpendicular transport (𝑘⊥) increases compared with the isotropic and globally anisotropic field models, the difference is enhanced as the local anisotropy is enhanced, and there is an order of magnitude increase in the ratio of perpendicular to parallel transport coefficients. We proposed a simple power spectrum synthesis method based on the Fourier analysis to extract the large and small scale power spectrum from any single space- craft observation with a long enough period and a high sampling frequency. We applied the method to the solar wind measurement by the magnetometer onboard the ACE spacecraft and reconstructed the large scale isotropic 2D spectrum and the small scale anisotropic 2D spectrum.