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| Titel |
Cascade rate and 3rd order structure functions in anisotropic turbulence |
| VerfasserIn |
Andrea Verdini, Roland Grappin, Petr Hellinger, Simone Landi, Wolf-Christian Müller |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111134
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| Publikation (Nr.) |
 EGU/EGU2015-11216.pdf |
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| Zusammenfassung |
| The measure of the third-order structure function, $\boldsymbol{Y}$, is employed in
the solar wind to compute the cascade rate of turbulence. In absence of a mean
field $B_0=0$, $\boldsymbol{Y}$ 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, $\boldsymbol{Y}$ is expected to
become more two dimensional (2D), that is, to have larger perpendicular
components, loosing the above simple symmetry. To get the cascade one should
compute the flux of $\boldsymbol{Y}$, which is not feasible with single-spacecraft
data, thus measurements rely upon assumptions about the unknown symmetry.
We use direct numerical simulations (DNS) of magneto-hydrodynamic (MHD)
turbulence to characterize the anisotropy of $\boldsymbol{Y}$. We find that for
strong guide field $B_0=5$ the degree of two-dimensionalization of $\boldsymbol{Y}$ depends on the
relative importance of shear and pseudo polarizations (the two components of an
Alfv\'en mode in incompressible MHD). The anisotropy also shows up in the
inertial range. The more turbulence 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 $B_0$ 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 DNS. This could be due to a weaker mean field and/or to solar wind
expansion. |
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