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The magnetic fields take place in different astrophysical objects, such as stars, planets, galaxies, accretion discs etc. Their evolution is usually described by the dynamo model. The equations of the dynamo are quite difficult to be solved, so different approximations are often used. One of the most successful ones are connected with the no- model which has been developed for thin discs that can be a good model for galaxies and accretion discs. In this paper we study the magnetic field evolution in the case of Keplerian rotation law. The results have been obtained for the simplest case and for the model with helicity fluxes. This approach can be useful while researching the role of convective flows in the discs. It is shown that the magnetic field can have oscillations in this case. Also we show the evolution of the energy which can significantly change because of the convective flows. [ABSTRACT FROM AUTHOR]

We investigate the saturation level of hydromagnetic turbulence driven by the magnetorotational instability in the case of vanishing net flux. Motivated by a recent paper of Bodo et al., we here focus on the case of a non-isothermal equation of state with constant thermal diffusivity. The central aim of the paper is to complement the previous result with closure parameters for mean-field dynamo models, and to test the hypothesis that the dynamo is affected by the mode of heat transport. We perform computer simulations of local shearing-box models of stratified accretion disks with approximate treatment of radiative heat transport, which is modeled via thermal conduction. We study the effect of varying the (constant) thermal diffusivity, and apply different vertical boundary conditions (BCs). In the case of impenetrable vertical boundaries, we confirm the transition from mainly conductive to mainly convective vertical heat transport below a critical thermal diffusivity. This transition is however much less dramatic when more natural outflow BCs are applied. Similarly, the enhancement of magnetic activity in this case is less pronounced. Nevertheless, heating via turbulent dissipation determines the thermodynamic structure of accretion disks and clearly affects the properties of the related dynamo. This effect, however, may have been overestimated in previous work, and a careful study of the role played by boundaries will be required. [ABSTRACT FROM AUTHOR]

The evolution of the magnetic field from the large-scale dynamo is considered a central feature of the accretion disk around a black hole. The resulting low-frequency oscillations introduced from the growth and decay of the field strength, along with the change in field orientation, play an integral role in the accretion disk behavior. Despite the importance of this process and how commonly it is invoked to explain variable features, it still remains poorly understood. We present a study of the dynamo using a suite of four global, high-resolution, MHD accretion disk simulations. We systematically vary the scale height ratio and find the large-scale dynamo fails to organize above a scale height ratio of h/r ≳ 0.2. Using spacetime diagrams of the azimuthal magnetic field, we show the large-scale dynamo is well ordered in the thinner accretion disk models, but fails to develop the characteristic “butterfly” pattern when the scale height ratio is increased, a feature which is also reflected in the power spectra. Additionally, we calculate the dynamo α-parameter and generate synthetic light curves. Using an emission proxy, we find the disks have markedly different characters as stochastic photometric fluctuations have a larger amplitude when the dynamo is unordered. [ABSTRACT FROM AUTHOR]