The spectral features of low-amplitude magnetic fluctuations in the solar wind and their comparison with moderate-amplitude fluctuations

The power-spectrum index of magnetic fluctuations is a crucial parameter for the characterization of nonlinear interactions affecting the solar wind turbulence. Most previous observations were made for large- and moderate-amplitude magnetic fluctuations, which include current sheets and intermittent structures. Here, by using measurements from the WIND spacecraft, we present magnetic spectral features as derived by the fast Fourier transform method from six-minute time series of magnetic fluctuations with low relative amplitudes ($\delta B/B_0$) of only 0.05–0.11. When comparing their spectra with those of moderate-amplitude fluctuations ($\delta B/B_0\approx 0.4$), we find that for the low-amplitude fluctuations the averaged magnetic spectral indices are $-1.67$ and $-1.46$, respectively, for small and large sampling angles, i.e., the angle between the Sun-to-Earth radial direction and the mean magnetic field direction. However, for the moderate-amplitude fluctuations, these two indices are $-1.89$ and $-1.70$, respectively. This result of the moderate-amplitude fluctuations is consistent with that of previous analyses, which, by using the wavelet technique, revealed spectral anisotropy of magnetic fluctuations and yielded an index of $-2$ in the parallel direction and $-5/3$ in the perpendicular direction. However, the result found here for the low-amplitude fluctuations has not been reported before, and thus will probably initiate new studies aiming to better understand the turbulent nature of such low-amplitude fluctuations. Future studies will help us to understand why the spectral anisotropy differs for fluctuations of different amplitudes.