  A phenomenology of isotropic magnetohydrodynamic turbulence subject to both
rotation and applied magnetic field is presented. It is assumed that the triple
correlations decay-time is the shortest between the eddy turn-over time and the
ones associated to the rotating frequency and Alfv\'en wave period. For $Pm=1$
it leads to four kinds of piecewise spectra, depending on the four parameters,
injection rate of energy, magnetic diffusivity, rotation rate and applied
field. With a shell model of MHD turbulence (including rotation and applied
magnetic field), spectra for $Pm \le 1$ are presented, together with the ratio
between magnetic and viscous dissipation.
