The anisotropy of magnetohydrodynamic Alfvenic turbulence

We perform direct three-dimensional numerical simulations for magnetohydrod ynamic (MHD) turbulence in a periodic box of size 2 pi threaded by strong u niform magnetic fields. We use a pseudospectral code with hyperviscosity an d hyperdiffusivity to solve the incompressible MHD equations. We analyze th e structure of the eddies as a function of scale. A straightforward calcula tion of anisotropy in wavevector space shows that the anisotropy is scale i ndependent. We discuss why this is not the true scaling law and how the cur vature of large-scale magnetic fields affects the power spectrum and leads to the wrong conclusion. When we correct for this effect, we find that the anisotropy of eddies depends on their size: smaller eddies are more elongat ed than larger ones along local magnetic held lines. The results are consis tent with the scaling law (k) over tilde(//) similar to (k) over tilde(perp endicular to)(2/3) recently proposed by Goldreich & Sridhar. Here (k) over tilde(//) (and (k) over tilde(perpendicular to)) are wavenumbers measured r elative to the local magnetic held direction. However, we see some systemat ic deviations that may be a sign of limitations to the model or our inabili ty to fully resolve the inertial range of turbulence in our simulations.