  A novel model of incompressible magnetohydrodynamic turbulence in the
presence of a strong external magnetic field is proposed for explanation of
recent numerical results. According to the proposed model, in the presence of
the strong external magnetic field, incompressible magnetohydrodynamic
turbulence becomes nonlocal in the sense that low frequency modes cause
decorrelation of interacting high frequency modes from the inertial interval.
It is shown that the obtained nonlocal spectrum of the inertial range of
incompressible magnetohydrodynamic turbulence represents an anisotropic
analogue of Kraichnan's nonlocal spectrum of hydrodynamic turbulence. Based on
the analysis performed in the framework of the weak coupling approximation,
which represents one of the equivalent formulations of the direct interaction
approximation, it is shown that incompressible magnetohydrodynamic turbulence
could be both local and nonlocal and therefore anisotropic analogues of both
the Kolmogorov and Kraichnan spectra are realizable in incompressible
magnetohydrodynamic turbulence.
