Solar wind turbulence anisotropy, from large to small scales.

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Abstract

Properties of solar wind fluctuations are often interpreted as those of a homogenous turbulent plasma, at MHD or ion scales. However solar-wind turbulence is not homogenous, being embedded in a spherically expanding flow of approximately constant speed.

We simulate turbulence in the solar wind by numerically integrating the Expanding Box Model (EBM) equations for MHD that retain the effect of spherical expansion and compute second order structure functions with respect to the local mean field.

We find that EBM simulations are able to reproduce the observed 3D anisotropy that differs from that one of homogenous turbulence. We explain the large-scale properties thanks to the component anisotropy (i.e. B_T,B_N>>B_R) that is driven by expansion and highlight that expansion effects persist all the way down to small scales (above the proton scale). 

However we also show that expansion effects are maximal when increments are computed along the radial axis, which is the axis of symmetry introduced by expansion and the only direction accessible to observations. Thus the observed properties can be only partially representative of solar wind turbulence.

We conclude by describing a project that uses both observations and simulations to uncover turbulence anisotropy in solar wind intervals in which expansion effects are expected to be minimal and so to understand whether small-scale anisotropy is controlled only by the magnetic field axis as in homogenous turbulence or by the radial axis as well.