Beyond the Maltese cross: geometry of turbulence between 0.2 and 1 AU

Abstract

The spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on the assumption of axisymmetry about the mean magnetic field, B0. However, several works indicate that this hypothesis might be partially wrong, thus raising two questions: (i) is it correct to interpret measurements at 1 AU (the so-called Maltese cross) in term of a sum of slab and 2D turbulence? (ii) what information is really contained in the Maltese cross? We solve direct numerical simulations of the MHD equations including the transverse stretching exerted by the solar wind flow and study the genuine 3D anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry about B0. We show that the evolution of the turbulent spectrum from 0.2 to 1 AU depends strongly on its initial anisotropy. An axisymmetric spectrum with respect to B0 keeps its axisymmetry, i.e., resists stretching perpendicular to radial, while an isotropic spectrum becomes essentially axisymmetric with respect to the radial direction. We conclude that close to the Sun, slow-wind turbulence has a spectrum that is axisymmetric around B0 and the measured 2D component at 1 AU describes the real shape of turbulent structures. On the contrary, fast-wind turbulence has a more isotropic spectrum at the source and becomes radially symmetric at 1 AU. Such structure is hidden by the symmetrization applied to the data that instead returns a slab geometry.

arXiv comment8 pages, 8 figures, accepted for publication in ApJ

References in corpus (14)

• Making the corona and the fast solar wind: a self-consistent simulation for the low-frequency Alfven waves from photosphere to 0.3AU
• Three-Dimensional Structure of Solar Wind Turbulence
• Coronal Heating, Weak MHD Turbulence and Scaling Laws
• Connecting the Sun and the Solar Wind: The First 2.5 Dimensional Self-consistent MHD Simulation under the Alfvén Wave Scenario
• On the origin of the 1/f spectrum in the solar wind magnetic field
• On weak and strong magnetohydrodynamic turbulence
• Turbulence in the sub-Alfvénic solar wind driven by reflection of low-frequency Alfvén waves
• Local Anisotropy, Higher Order Statistics, and Turbulence Spectra
• Scaling law for the heating of solar coronal loops
• Evolution of turbulence in the expanding solar wind, a numerical study
• Imprints of expansion onto the local anisotropy of solar wind turbulence
• Parametric decay of parallel and oblique Alfven waves in the expanding solar wind
• Coronal heating in coupled photosphere-chromosphere-coronal systems: turbulence and leakage
• Transition from weak to strong cascade in MHD turbulence

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• Anisotropies of the magnetic field fluctuations at kinetic scales in the solar wind : Cluster observations
• Three-dimensional local anisotropy of velocity fluctuations in the solar wind
• Comparing turbulent cascades and heating vs spectral anisotropy in solar wind via direct simulations