Reexamination of data analysis for −2 spectral index at small θVB angle

Abstract

The power spectrum of magnetic field in the solar wind turbulence was reported to be anisotropic with respect to the angle (θV B) between local mean magnetic field (B0) and sampling direction. When the local B0 is nearly parallel (perpendicular) to the sampling direction, the spectral index was found to be close to −2 (−5/3). This result was widely considered as observational evidence for the critical balance theory. However, it was also found that after intermittent structures were removed, the spectral index in the parallel direction turned out to be about −1.6, and the anisotropy disappeared. Here, we present a method to guarantee a nearly constant θV B in each local time window that is mainly influenced by the mother function of the Morlet wavelet. Then we reexamine the anisotropy of the spectral index of the magnetic field, by using the observations from WIND spacecraft in the fast solar wind. Our result shows that the −2 spectral index at the parallel angular bin θV B < 6◦ disappears, and the spectr...