Abstract
Abstract The self-correlation level contours at 1010 cm scale reveal a 2D isotropic feature in both the slow solar wind fluctuations and the fast solar wind fluctuations. However, this 2D isotropic feature is obtained based on the assumption of axisymmetry with respect to the mean magnetic field. Whether the self-correlation level contours are still 3D isotropic remains unknown. Here we perform for the first time a 3D self-correlation level contours analysis on the solar wind turbulence. We construct a 3D coordinate system based on the mean magnetic field direction and the maximum fluctuation direction identified by the minimum-variance analysis method. We use data with 1 hr intervals observed by WIND spacecraft from 2005 to 2018. We find, on one hand, in the slow solar wind, the self-correlation level contour surfaces for both the magnetic field and the velocity field are almost spherical, which indicates a 3D isotropic feature. On the other hand, there is a weak elongation in one of the perpendicular directions in the fast solar wind fluctuations. The 3D feature of the self-correlation level contours surfaces cannot be explained by the existing theory.
Highlights
The magnetohydrodynamic (MHD) turbulence exhibits anisotropic features as a result of the preferred direction that the background magnetic field determines (Shebalin et al 1983)
We construct a 3-D coordinate system based on the mean magnetic field direction and the maximum fluctuation direction identified by the minimum-variance analysis (MVA) method
The solar wind is observed to be in a turbulence state (Tu & Marsch 1995) and various studies proposed that solar wind turbulence is 2-D anisotropic based on theories (Oughton et al 1994; Goldreich & Sridhar 1995), simulations (Cho & Vishniac 2000) and observations related to the power spectral index (Horbury et al 2008; Podesta 2009; Chen et al 2010; Wicks et al 2010; He et al 2013), structure function (Luo & Wu 2010) and correlation function (Matthaeus et al 1990; Dasso et al 2005)
Summary
The magnetohydrodynamic (MHD) turbulence exhibits anisotropic features as a result of the preferred direction that the background magnetic field determines (Shebalin et al 1983). The Maltese cross is one 2-D pioneering work It consists of two lobes, one elongated along to the mean field direction (slab-like fluctuations), and the other elongated along the perpendicular direction to the mean field direction (2-D fluctuations) and is obtained by a 2-D self-correlation analysis (Matthaeus et al 1990). Dasso et al (2005) applied the correlation function method by using two-day-long data from Advanced Composition Explorer (ACE) spacecraft for the slow solar wind and the fast solar wind separately. They find that the fast wind mainly contains slab-like fluctuations and the slow wind 2-D fluctuations.
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