Influence of Stationary Local Mean Field on the Scaling Anisotropy in MHD 1 Turbulence

Magnetohydrodynamic (MHD) turbulence is revealed to have scaling anisotropy based on a local mean magnetic field, which is usually calculated through the average of the magnetic field vectors at two points. However, the thus reconstructed parallel spectrum can be contaminated by intermittent events and nonparallel fluctuations. In this work, we simulate the driven compressible MHD turbulence to investigate the influence of the stationary local mean field on the scaling anisotropy. By examining whether the local parallel samplings are along the magnetic field directions, we confirm that the stationary local mean field can be fulfilled by the criterion ϕ < 10◦ with ϕ being the angle between the two local mean magnetic fields after cutting the sampled magnetic field intervals into two halves. The three-dimensional (3D) numerical data also show that the samplings with the stationary local mean fields successfully prevent intermittency events and nonparallel fluctuations from mixing into the measured parallel fluctuations. With the true parallel samplings achieved, the local parallel spectra of the magnetic field and the velocity are found to be closer to -5/3 scaling than -2 scaling. The requirement of the stationary local mean fields has nearly no effects on the perpendicular scaling, which has a about -5/3 power-law index. Among the removed intermittent events from the local parallel samplings, one third of the events are identified as TDs, one third as RDs, and the rest as EDs. The samplings with the nonstationary local mean fields display large separations between the field directions and the parallel directions, about two times of those with the stationary local mean fields according to statistical analyses. These results have important implications for interpreting the nonlinear interactions of MHD turbulence.