We investigate the influence of a uniform magnetic field ${\mathbf{B}}_0=B_0{\mathbf{e}}_{\parallel }$ on energy decay laws in incompressible magnetohydrodynamic (MHD) turbulence. The nonlinear transfer reduction along ${\mathbf{B}}_0$ is included in a model that distinguishes parallel and perpendicular directions, following a phenomenology of Kraichnan. We predict a slowing down of the energy decay due to anisotropy in the limit of strong $B_0$, with distinct power laws for energy decay of shear- and pseudo-Alfvén waves. Numerical results from the kinetic equations of Alfvén wave turbulence recover these predictions, and MHD numerical results clearly tend to follow them in the lowest perpendicular planes.

• Received 21 June 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.074502