  A heuristic model is given for anisotropic magnetohydrodynamics (MHD)
turbulence in the presence of a uniform external magnetic field $B_0 {\bf {\hat
e}_{\pa}}$. The model is valid for both moderate and strong $B_0$ and is able
to describe both the strong and weak wave turbulence regimes as well as the
transition between them. The main ingredient of the model is the assumption of
constant ratio at all scales between \add{the} linear wave period and \add{the}
nonlinear turnover timescale. Contrary to the model of critical balance
introduced by Goldreich and Sridhar [P. Goldreich and S. Sridhar, ApJ {\bf
438}, 763 (1995)], it is not assumed in addition that this ratio be equal to
unity at all scales which allows us to use the Iroshnikov-Kraichnan
phenomenology. It is then possible to recover the widely observed anisotropic
scaling law $\kpa \propto \kpe^{2/3}$ between parallel and perpendicular
wavenumbers (with reference to $B_0 {\bf {\hat e}_{\pa}}$) and to obtain the
universal prediction, $3\alpha + 2\beta = 7$, for the total energy spectrum
$E(\kpe,\kpa) \sim \kpe^{-\alpha} \kpa^{-\beta}$. In particular, with such a
prediction the weak Alfv\'en wave turbulence constant-flux solution is
recovered and, for the first time, a possible explanation to its precursor
found numerically by Galtier et al [S. Galtier et al., J. Plasma Phys. {\bf
63}, 447 (2000)] is given.

Plasma Physics;Fluid Dynamics