The possibility of explaining shear flow dynamos by a magnetic shear-current (MSC) effect is examined via numerical simulations. Our primary diagnostics is the determination of the turbulent magnetic diffusivity tensor η. In our setup, a negative sign of its component ηyx is necessary for coherent dynamo action by the SC effect. To be able to measure turbulent transport coefficients from systems with magnetic background turbulence, we present an extension of the test-field method (TFM), which we call the nonlinear TFM (NLTFM). We study two different cases: One where the pressure gradient is dropped, which is generally referred to Burgulence, and the corresponding momentum equation as Burger's equation. In this case the NLTFM simplifies, as additional equations for the density fluctuations do not need to be solved for. In another case, we excite magnetic background turbulence by kinetic forcing only, in a regime, where it drives a small-scale dynamo instability. This case requires the full NLTFM problem to be solved, and is numerically much more challenging than its simplified counterparts.
In the Burgulence problem, when we force only kinetically, negative ηyx are obtained with exponential growth in both the radial and azimuthal mean magnetic field components. Using magneto-kinetic forcing, the field growth is no longer exponential, while NLTFM yields positive ηyx. By employing an alternative forcing from which wavevectors whose components correspond to the largest scales are removed, the exponential growth is recovered, but the NLTFM results do not change significantly. Moving into the full MHD problem, large-scale dynamo action is seen to require large magnetic Prandtl numbers, and the NLTFM measurements all give consistently insignificant or positive ηyx measurements when shear rate and magnetic Prandtl number are varied. Analyzing the dynamo excitation conditions for the coherent SC and incoherent α and SC effects shows that the incoherent effects are the main drivers of the dynamo in the majority of cases. We find no evidence for MSC-effect-driven dynamos in our simulations.
The first 10-20 min will be presented by Matthias Rheinhardt (Aalto U).
