On the weakly anisotropic nature of the time-stationary turbulence in the solar wind

At the frequency range from 0.01Hz to 0.1Hz, the power spectrum of the fluctuations in the solar wind turbulence was recently observed to be anisotropic with respect to the direction of local mean magnetic field (B₀). These observations are considered as evidence for a "critical balance" style cascade. However, we find that the anisotropy of the spectral index seems to be very weak, if we use continuous time series which are time-stationary and have nearly constant local B₀. We apply the fast fourier transform (FFT) on these time series selected from the eight-year magnetic field (B) and flow velocity (V) data observed by the WIND spacecraft in the high-speed solar wind. Our results show that the FFT spectral indices of the time series with B₀ nearly parallel and perpendicular to the Sun-To-Earth radial direction are not significantly different. This work provides new clues on the nature of the anisotropy of the solar wind turbulence and thus will improve our understanding of the turbulent energy cascade.