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The scaling anisotropy is crucial to interpret the nonlinear interactions insolar wind turbulence. Previous observations provide diverse results and thestructure function analyses are also reported to be an approach to investigatethe scaling anisotropy based on a local magnetic field. However, thedetermination of the sampling angle with respect to the local backgroundmagnetic field requires that the observed time series for the average are timestationary. Whether or not this required time stationarity is compatible withthe measurements has not been investigated. Here we utilize the second-orderstructure function method to study the scaling anisotropy with atime-stationary background field.We analyze 88 fast solar wind intervals eachwith time durations larger than 2 days measured by WIND spacecraft in theperiod 2005-2018. We calculate the local magnetic field as the average of thetime series B whose time stationarity is fulfilled by our criterion phi smallerthan 10 deg (phi is the angle between the two averaged magnetic fields aftercutting B into two halves). We find for the first time the isotropic scalingfeature of the magnetic-trace structure functions with scaling indices -0.63 pm0.08 and -0.70 pm 0.04, respectively, with the local magnetic field paralleland perpendicular to the solar wind velocity directions. The scaling for thevelocity-trace structure functions is also isotropic and the indices are -0.47pm 0.10 and -0.51 pm 0.09. We also find that with increasing phi threshold to90 deg, the scaling index of the magnetic-trace structure function in theparallel direction decreases to -0.81, while the rms of the instantaneous anglebetween magnetic field and solar wind velocity increases up to 45deg at thetimescale 150 s, indicating a mix of perpendicular measurements into parallelones at large scales.

Isotropic Scaling Features Measured Locally in the Solar Wind Turbulence with Stationary Background Field