MHD Simulation Attempts to Explain the Spectral Symmetry and Minimum Variance Directions in the Solar Wind

Solar wind fluctuations are known to be anisotropic, both in wave vector and in field fluctuation directions. Various sets of two-spacecraft observations, in addition to ensembles of correlations from a single spacecraft, show there is some preference for wave vector directions nearly perpendicular to the mean magnetic field. Many studies have also shown that magnetic field (and to a lesser extent the velocity field) has a minimum variance direction typically along the mean magnetic field direction, and that this persists even as the mean field turns to nearly perpendicular to the radial in the outer heliosphere. The requirement that the wave vectors and fluctuations must turn with the mean field eliminates simple views of planar Alfvén waves or quasi-two-dimensional fluctuations or superpositions of these. In previously reported simulation work, we have shown that inhomogeneity transverse to the radial flow direction as well as nonlinear interactions are required to explain the observations. We now have added to our simulation code non-radial wave vectors to the inflow population of waves, as well as three-dimensional microstreams (radial flows that depend on both transverse directions). This paper will explore the extent to which these additions aid in solving the anisotropy and minimum variance problems. Preliminary results are similar to those found with 2-D simulations which showed some turning of wave vectors due to shear, but as yet no clear minimum variance signature. We will use higher resolution simulations to determine if part of the problem is that the scale of our assumed population of fluctuations is too large.