The isotropic nature of the background turbulence spectra in the solar wind

Abstract:

At the high-frequency end of the inertial range, the solar wind turbulence power spectrum was recently found to be anisotropic with respect to the direction of local magnetic field, as an evidence for the presence of a “critical balance” style turbulence cascade. However, we find that the spectral anisotropy seems to result from intermittent structures. The following two independent studies corroborate this statement by showing that the power spectra of the background turbulence, in which there are no intermittent structures, have an isotropic nature. In Study 1, we remove the wavelet coefficients of the local intermittency with large partial variance increment (PVI), and find the spectral indices of the magnetic field are 1.63±0.02, independent of the angle θ_RB between the direction of the local background magnetic field and the radial direction. In Study 2, we make a statistical study on the magnetic field spectral indices obtained by using Fast Fourier Transform on 40 time series, in which no intermittent structures appear. We find that for the time series with 0^o<θ_RB <6^o, the probability distribution of the observed spectral indices peaks at -1.7, while the -2 index predicted by the “critical balance” theory rarely appears. For the time series with 84 ^o <θ_RB <90 ^o, the probability distribution of the indices peaks at -1.5. Considering the uncertainty of the statistics, these results show that the background-turbulence spectra are nearly isotropic with respect to θ_RB, which may be consistent with some explanations based on hydrodynamic turbulence theory.