Anisotropy of Self-Correlation Level Contours in Three-Dimensional Magnetohydrodynamic Turbulence

Abstract

MHD turbulence is considered to be anisotropic owing to the presence of a magnetic field, and its self-correlation anisotropy has been unveiled by solar wind observations. Here, based on numerical results of compressible MHD turbulence with a global mean magnetic field, we explore variations of the normalized self-correlation function’s (NCF) level contours with the scale as well as their evolution. The analyses reveal that the NCF’s level contours tend to elongate in the direction parallel to the mean magnetic field, and the elongation becomes weak with decreasing intervals. These results are consistent with slow solar wind observations. The less anisotropy of the NCF’s level contours with the shorter intervals can be produced by the fact that coherent structures stretch more along the parallel direction at the long intervals than at the short intervals. The analyses also disclose that as the simulation time builds up, the NCF’s level contours change thinner and thinner, and the anisotropy of the NCF’s level contours grows, which can be caused by the break of large coherent structures into small ones. The increased self-correlation anisotropy with time foretells that the self-correlation anisotropy of solar wind turbulence enlarges with the radial distance, which needs to be tested against observations by using Parker Solar Probe (PSP) measurements.