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

TL;DR

This paper uses 3D global MHD simulations to study the evolution of thick accretion flows, emphasizing the importance of azimuthal extent for turbulence and dynamo processes, and revealing the role of dynamical quenching.

Contribution

It demonstrates the convergence criteria for simulations of thick accretion flows and explores the effects of azimuthal extent and dynamo quenching in a global context.

Findings

Convergence achieved with 42 grid points per scale height.

Azimuthal extent of at least π/2 needed to capture flow structure.

Dynamo cycle is intermittent with significant alpha-quenching effects.

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 $\langle \langle Q_{\theta} \rangle \rangle \langle \langle Q_{\phi} \rangle \rangle \geq 300$ 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 $\geq \pi/2$ 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.