3D global simulations of RIAFs: convergence, effects of azimuthal extent and dynamo

Abstract

We study the long-term evolution of non-radiative geometrically thick ($H/R\approx 0.5$) accretion flows using 3D global ideal MHD simulations and a pseudo-Newtonian gravity. We find that resolving the scale height with 42 grid points is adequate to obtain convergence with the product of quality factors $⟨⟨Q_{\theta }⟩⟩⟨⟨Q_{\varphi }⟩⟩\ge 300$ and magnetic tilt angle ${\theta }_B\sim 13^{\circ }-14^{\circ }$. Like previous global isothermal thin disk simulations, we find stronger mean magnetic fields for the restricted azimuthal domains. Imposing periodic boundary conditions with the azimuthal extent smaller than $2\pi$ make the turbulent field at low $m$ appear as a mean field in the runs with smaller azimuthal extent. But unlike previous works, we do not find a monotonic trend in turbulence with the azimuthal extent. We conclude that the minimum azimuthal extent should be $\ge \pi /2$ to capture the flow structure, but a full $2\pi$ extent is necessary to study the dynamo. We find an intermittent dynamo cycle, with $\alpha$-quenching playing an important role in the nonlinear saturated state. Unlike previous local studies, we find almost similar values of kinetic and magnetic $\alpha$-s, giving rise to an irregular distribution of dynamo-$\alpha$. The effects of dynamical quenching are shown explicitly for the first time in global simulations of accretion flows.

arXiv comment24 pages, 27 figures, 4 tables. Accepted in MNRAS. We welcome comments and suggestions

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