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Article Dans Une Revue The Astrophysical Journal Année : 2015

Anisotropy of Third-order Structure Functions in MHD Turbulence

Andrea Verdini
  • Fonction : Auteur
Roland Grappin
P. Hellinger
  • Fonction : Auteur
Simone Landi
  • Fonction : Auteur
Wolf-Christian Müller
  • Fonction : Auteur

Résumé

The measure of the third-order structure function, \boldsymbolY , is employed in the solar wind to compute the cascade rate of turbulence. In the absence of a mean field B0=0, \boldsymbolY is expected to be isotropic (radial) and independent of the direction of increments, so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind, \boldsymbolY is expected to become more two-dimensional (2D), that is, to have larger perpendicular components, losing the above simple symmetry. To get the cascade rate, one should compute the flux of \boldsymbolY , which is not feasible with single-spacecraft data thus, measurements rely on assumptions about the unknown symmetry. We use direct numerical simulations (DNSs) of magnetohydrodynamic (MHD) turbulence to characterize the anisotropy of \boldsymbolY . We find that for strong guide field B0=5 the degree of two-dimensionalization depends on the relative importance of shear-Alfvén and pseudo-Alfvén polarizations (the two components of an Alfvén mode in incompressible MHD). The anisotropy also shows up in the inertial range. The more \boldsymbolY is 2D, the more the inertial range extent differs along parallel and perpendicular directions. We finally test the two methods employed in observations and find that the so-obtained cascade rate may depend on the angle between B0 and the direction of increments. Both methods yield a vanishing cascade rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar wind turbulence compared to our DNSs. This could be due to a weaker mean field and/or to solar wind expansion.

Dates et versions

hal-01552005 , version 1 (30-06-2017)

Identifiants

Citer

Andrea Verdini, Roland Grappin, P. Hellinger, Simone Landi, Wolf-Christian Müller. Anisotropy of Third-order Structure Functions in MHD Turbulence. The Astrophysical Journal, 2015, 804, pp.119. ⟨10.1088/0004-637X/804/2/119⟩. ⟨hal-01552005⟩
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