On the nature of magnetic turbulence in rotating, shearing flows

The local properties of turbulence driven by themagnetorotational instability (MRI) in rotating, shearing flows are studied in the framework of a shearing-box model. Based on numerical simulations, we propose that the MRI-driven turbulence comprises two components: the large-scale shear-aligned strong magnetic field and the small-scale fluctuations resembling magnetohydrodynamic (MHD) turbulence. The energy spectrum of the large-scale component is close to k(-2), whereas the spectrum of the small-scale component agrees with the spectrum of strong MHD turbulence k(-3/2). While the spectrum of the fluctuations is universal, the outerscale characteristics of the turbulence are not; they depend on the parameters of the system, such as the net magnetic flux. However, there is remarkable universality among the allowed turbulent states -their intensity v(0) and their outer scale lambda(0) satisfy the balance condition v(0)/lambda(0) similar to d Omega/dln r, where d Omega/dln r is the local orbital shearing rate of the flow. Finally, we find no sustained dynamo action in the Pm = 1 zero net-flux case for Reynolds numbers as high as 45 000, casting doubts on the existence of an MRI dynamo in the Pm <= 1 regime.