  The magnetorotational (MRI) dynamo has long been considered one of the
possible drivers of turbulent angular momentum transport in astrophysical
accretion disks. However, various numerical results suggest that this dynamo
may be difficult to excite in the astrophysically relevant regime of magnetic
Prandtl number (Pm) significantly smaller than unity, for reasons currently not
well understood. The aim of this article is to present the first results of an
ongoing numerical investigation of the role of both linear and nonlinear
dissipative effects in this problem. Combining a parametric exploration and an
energy analysis of incompressible nonlinear MRI dynamo cycles representative of
the transitional dynamics in large aspect ratio shearing boxes, we find that
turbulent magnetic diffusion makes the excitation and sustainment of this
dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for
decreasing Pm. This results in an increase in the critical Rm of the dynamo for
increasing kinematic Reynolds number (Re), in agreement with earlier numerical
results. Given its very generic nature, we argue that turbulent magnetic
diffusion could be an important determinant of MRI dynamo excitation in disks,
and may also limit the efficiency of angular momentum transport by MRI
turbulence in low Pm regimes.
