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

• Published in: Journal of Geophysical Research: Space Physics 2012-05, Vol.117 (A5), p.n/a
• Main Author: Borovsky, Joseph E.
• Format: Article
• Language: English
• Subjects: Astrophysics; Classification; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fluctuations; Magnetic fields; magnetohydrodynamics; periodogram; Planetology; solar wind; Space; Statistical analysis; turbulence; variance
• ISSN: 0148-0227; 2169-9380; 2156-2202; 2169-9402
• DOI: 10.1029/2011JA017499

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, Alfvén 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. Key Points Properties of MHD turbulence vary with plasma properties Coronal‐hole versus non‐coronal‐hole more important than fast versus slow Density of current sheets is important