Influence of Large-scale Field Structures on the Scaling Anisotropy in 3D MHD Turbulence

Magnetohydrodynamic (MHD) turbulence is revealed to have scaling anisotropy based on structure function calculations. Recent studies on solar wind turbulence found that the scaling anisotropy disappears when removing large-scale field structures. This finding raises questions as to whether numerical MHD turbulences have large-scale field structures. How do these structures affect the scaling anisotropy therein? Here we investigate these questions with a driven compressible three-dimensional MHD turbulence. We introduce a new method to check how the random stationarity condition is satisfied. We find for the first time in the numerical MHD turbulence that the large-scale field structures destroy the random stationarity of the local fields and make samplings nonparallel to the instantaneous fields be calculated as apparent parallel samplings. This mixture makes statistical calculations show anisotropic scaling of the turbulence. When we select only the random stationary data intervals, the statistical results show an isotropic nature. We also find that among the large-scale field structures, one-third are tangential discontinuities (TDs), one-third are rotational discontinuities (RDs), and the rest are EDs (either TD or RD). These results show that the large-scale structures in the numerical MHD turbulence have important influence on the structure function analysis.