Low-Amplitude Solar Wind Turbulence: Spectral Features

Abstract:

Most previous turbulence observations in the solar wind were made for large- and moderate-amplitude fluctuations, which include current sheets and intermittent structures. Here we first present the spectral analysis on the low-amplitude fluctuations that may be considered as nearly stationary, and compare the results with those of moderate-amplitude fluctuations which show intermittency. By using measurements from the WIND spacecraft, we present spectral features as derived by the FFT method from 6-minute time series of magnetic fluctuations with low relative amplitudes of only 0.05 to 0.11. When comparing their spectra with those of moderate-amplitude fluctuations (relative amplitude of about 0.4), we find that for the low-amplitude fluctuations the averaged 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 and yielded an index of -2 in the parallel direction and -5/3 in the perpendicular direction. But the result found here for the low-amplitude fluctuations was not reported before, and thus will probably initiate new studies aiming to understand better the turbulence nature of such low-amplitude fluctuations. Future studies will help us to understand why the spectral anisotropy differs for fluctuations of different amplitudes.