The velocity and magnetic field fluctuations of the solar wind at 1 AU: Statistical analysis of Fourier spectra and correlations with plasma properties

摘要

Ten years of ACE plasma and magnetic field measurements are divided into 20,076 subintervals that are 4.55 h long. Each subinterval is Fourier analyzed resulting in a statistical ensemble of 20,076 realizations of the solar wind and its "turbulence." Oxygen charge-state ratios are used to categorize each subinterval as coronal-hole-origin plasma, non-coronal-hole-origin plasma, or ejecta. A number of known properties of the solar wind v and B fluctuations are statistically confirmed and new informatin as functions of the type of plasma is obtained. For the fluctuations it is found that the coronal-hole-origin versus non-coronal-hole-origin classification is more fundamental than a fast-wind versus slow-wind classification. In the frequency range 4.3 × 10~(-4)-1.9 × 10~(-3) Hz, the ensemble the mean spectral indices of the magnetic field, velocity, and total energy are -1.62, -1.41, and -1.52, however the spectral indices vary with changes in the type of plasma. The number of strong current sheets in each subinterval is recorded. The fluctuation amplitudes, Alfven ratios, and outward-inward Elsasser ratios are all strongly correlated with the properties of the plasma and the density of current sheets. Regions wherein the fluctuation spectra are shallowest correspond to coronal-hole plasma; regions wherein the spectra are steepest correspond to non-coronal-hole plasma and ejecta. The autocorrelation times for the spectral indices and amplitudes are 20-30 h, similar to the autocorrelation times for the proton specific entropy, the carbon charge-state ratio, the density of strong current sheets, and the classification of plasma. Analysis is performed to interpret ensembles of spectra with variance error.