  Using the magnetohydrodynamic (MHD) description, we develop a nonlinear
dynamo model that couples the evolution of the large scale magnetic field with
turbulent dynamics of the plasma at small scale by electromotive force (e.m.f.)
in the induction equation at large scale. The nonlinear behavior of the plasma
at small scale is described by using a MHD shell model for velocity field and
magnetic field fluctuations.The shell model allow to study this problem in a
large parameter regime which characterizes the dynamo phenomenon in many
natural systems and which is beyond the power of supercomputers at today. Under
specific conditions of the plasma turbulent state, the field fluctuations at
small scales are able to trigger the dynamo instability. We study this
transition considering the stability curve which shows a strong decrease in the
critical magnetic Reynolds number for increasing inverse magnetic Prandlt
number $\textrm{Pm}^{-1}$ in the range $[10^{-6},1]$ and slows an increase in
the range $[1,10^{8}]$. We also obtain hysteretic behavior across the dynamo
boundary reveling the subcritical nature of this transition. The system,
undergoing this transition, can reach different dynamo regimes, depending on
Reynolds numbers of the plasma flow. This shows the critical role that the
turbulence plays in the dynamo phenomenon. In particular the model is able to
reproduce the dynamical situation in which the large-scale magnetic field jumps
between two states which represent the opposite polarities of the magnetic
field, reproducing the magnetic reversals as observed in geomagnetic dynamo and
in the VKS experiments.
