| Authors Affiliation: | 1 JILA, University of Colorado and National Institute of Standards and Technology, 440 UCB, Boulder, CO 80309-0440, USA
2 Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
3 IUCAA, Post Bag 4, Ganeshkhind, Pune 411007, India
4 Department of Physics, Ashoka University, Rajiv Gandhi Education City, Rai, Sonipat 131029, Haryana, INDIA |
| Abstract : | Magnetorotational instability (MRI) is proposed to be responsible for angular momentum transport in a rotationally supported sufficiently ionised accretion disc. Numerical simulations suggest that MRI-mediated dynamo can generate the large-scale magnetic field, a key ingredient in producing jets/outflows in the accreting systems. We study the MRI dynamo in a geometrically thin disc (H/R ≪ 1) using stratified zero net flux (ZNF) shearing box simulations. In our simulation, large-scale magnetic fields and EMFs show diverse periodicities not observed in the previous studies. Further, we adopt a novel inversion algorithm called the ‘Iterative Removal Of Sources’ (IROS) to extract the turbulent dynamo coefficients in the mean-field closure using the shearing box simulation data. We show that an α−effect is predominantly responsible for creating the poloidal field from the toroidal field. At the same time, shear generates back a toroidal field from the poloidal field, indicating that an α − Ω-type dynamo is operative in MRI-driven accretion discs. We find encouraging evidence that a generative helicity flux is responsible for the effective α-effect. We also find that strong outflow and turbulent pumping transport large-scale magnetic fields away from the disc mid-plane. They are the principal mechanisms for diffusing the large-scale magnetic field instead of the turbulent diffusivity in our simulation. |
