Non-axisymmetric Anisotropy of the Inertial Range in the Turbulent Solar Wind

Details Non-axisymmetric Anisotropy of the Inertial Range in the Turbulent Solar Wind Non-axisymmetric Anisotropy of the Inertial Range in the Turbulent Solar Wind

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh43c1978t&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SH43C-1978

Physics

[2164] Interplanetary Physics / Solar Wind Plasma

Scientific paper

Anisotropy is a key topic for theoretical, numerical and observational studies of plasma turbulence in the solar wind. However, fluctuations in the solar wind are found to be ordered wrt both the background field and flow directions, for example Belcher and Davis found that the fluctuations on average have 5 : 4 : 1 power anisotropy in an orthogonal coordinate system whose axis are [eBxeR, eBx(eBxeR), eB], where eB is a unit vector in the direction of the average magnetic field and eR is a unit vector in the direction radially away from the sun. The fact that the fluctuations are not axisymmetric wrt the background field is paradoxical in the context of idealized models for MHD turbulence. Here, we show that this observed non-axisymmetric anisotropy may arise as a data sampling effect rather than as a result of the physical properties of the turbulence. We will quantify the observed non-axisymmetry via the Power spectral Density ratio of eBxeR : eBx(eBxeR) for in-situ measurements in fast wind. The observed non-axisymmetric ratio will then be compared with Direct Numerical Simulations that are sampled with a "fly through", emulating in-situ single spacecraft observations using Taylor's hypothesis with a constant velocity. We will see this "fly through" model is sufficient to reproduce the observed non-axisymmetric anisotropy in the inertial range of the solar wind. Thus, the observed non-axisymmetric anisotropy may simply arise as a sampling effect related to the one-dimensional spectrum and Taylor's hypothesis. A simple analytical model for a field composed of a linear superposition of transverse waves confirms this result. Prospects for constructing statistical measures, such as structure functions, in space using super-ensembles of multi-spacecraft observations will be discussed.

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