Large scale dynamos with ambipolar diffusion nonlinearity

It is shown that ambipolar diffusion as a useful model for nonlinearity lea ds to similar behaviour of large scale turbulent dynamos as full MHD. This is demonstrated using both direct simulations in a periodic box and a closu re model for the magnetic correlation functions applicable to infinite spac e. Large scale fields develop via a nonlocal inverse cascade as described b y the ct-effect. However, magnetic helicity can only change on a resistive timescale, so the time it takes to organize the field into large scales inc reases with magnetic Reynolds number.