Spectrum of MHD Turbulence*

We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence of a large-scale external magnetic field. We argue that in the inertial range of scales, magnetic-field and velocity-field fluctuations tend to align the directions of their polarizations. However, in driven turbulence the perfect alignment cannot be reached, it is precluded by the presence of a constant energy flux over scales. As a consequence, the directions of fluid and magnetic-field fluctuations at each scale~$\lambda$ become aligned within the angle~$\phi_{\lambda}\propto \lambda^{1/4}$, which leads to scale-dependent depletion of nonlinear interaction and to the field-perpendicular energy spectrum~$E(k_{\perp})\propto k_{\perp}^{-3/2}$. Our results may be universal, i.e., independent of the external magnetic field, since small-scale fluctuations locally experience a strong field produced by large-scale eddies. The application of the results to diffractive interstellar scintillation is discussed.