  We study the anisotropy of II-order structure functions defined in a frame
attached to the local mean field in three-dimensional (3D) direct numerical
simulations of magnetohydrodynamic turbulence, including or not the solar wind
expansion. We simulate spacecraft flybys through the numerical domain by taking
increments along the radial (wind) direction that forms an angle of $45^o$ with
the ambient magnetic field. We find that only when expansion is taken into
account, do the synthetic observations match the 3D anisotropy observed in the
solar wind, including the change of anisotropy with scales. Our simulations
also show that the anisotropy changes dramatically when considering increments
oblique to the radial directions. Both results can be understood by noting that
expansion reduces the radial component of the magnetic field at all scales,
thus confining fluctuations in the plane perpendicular to the radial. Expansion
is thus shown to affect not only the (global) spectral anisotropy, but also the
local anisotropy of second-order structure functions by influencing the
distribution of the local mean field, which enters this higher-order
statistics.
