Magnetorotational instability driven dynamos at low magnetic Prandtl numbers

7 pages, 7 figures, submitted to MNRAS. Version with higher resolution figures is available at http://www.helsinki.fi/~kapyla/publ.htmlNumerical simulations of the magnetorotational instability (MRI) with zero initial net flux in a non-stratified isothermal cubic domain are used to demonstrate the importance of magnetic boundary conditions. In fully periodic systems the level of turbulence generated by the MRI strongly decreases as the magnetic Prandtl number (Pm), which is the ratio of kinematic viscosity and magnetic diffusion, is decreased. No MRI or dynamo action below Pm=1 is found, agreeing with earlier investigations. Using vertical field conditions, which allow magnetic helicity fluxes out of the system, the MRI is found to be excited in the range 0.1Peer reviewe