Evolution of turbulence in the expanding solar wind, a numerical study

Abstract

We study the evolution of turbulence in the solar wind by solving numerically the full 3D magneto-hydrodynamic (MHD) equations embedded in a radial mean wind. The corresponding equations (expanding box model or EBM) have been considered earlier but never integrated in 3D simulations. Here, we follow the development of turbulence from 0.2 AU up to about 1.5 AU. Starting with isotropic spectra scaling as $k^{-1}$, we observe a steepening toward a $k^{-5/3}$ scaling in the middle of the wavenumber range and formation of spectral anisotropies. The advection of a plasma volume by the expanding solar wind causes a non-trivial stretching of the volume in directions transverse to radial and the selective decay of the components of velocity and magnetic fluctuations. These two effects combine to yield the following results. (i) Spectral anisotropy: gyrotropy is broken, and the radial wavevectors have most of the power. (ii) Coherent structures: radial streams emerge that resemble the observed microjets. (iii) Energy spectra per component: they show an ordering in good agreement with the one observed in the solar wind at 1 AU. The latter point includes a global dominance of the magnetic energy over kinetic energy in the inertial and $f^{-1}$ range and a dominance of the perpendicular-to-the-radial components over the radial components in the inertial range. We conclude that many of the above properties are the result of evolution during transport in the heliosphere, and not just the remnant of the initial turbulence close to the Sun.

14 pages, 12 figures, accepted in ApJ

References in corpus (3)

• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• Turbulence in the sub-Alfvénic solar wind driven by reflection of low-frequency Alfvén waves
• Diamagnetic and Expansion Effects on the Observable Properties of the Slow Solar Wind in a Coronal Streamer

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• Anisotropy of third-order structure functions in MHD turbulence
• Solar wind turbulence from MHD to sub-ion scales: high-resolution hybrid simulations
• Beyond the Maltese cross: geometry of turbulence between 0.2 and 1 AU
• Reconnection-Driven Magnetohydrodynamic Turbulence in a Simulated Coronal-Hole Jet
• Three-dimensional local anisotropy of velocity fluctuations in the solar wind
• Sub-structure formation in starless cores
• Simulations of Energetic Particles Interacting with Nonlinear Anisotropic Dynamical Turbulence