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Strongly magnetized accretion discs around black holes have attractivefeatures that may explain enigmatic aspects of X-ray binary behaviour. Thestructure and evolution of these discs are governed by a dynamo-like mechanism,which channels part of the accretion power liberated by the magnetorotationalinstability (MRI) into an ordered toroidal magnetic field. To study dynamoactivity, we performed three-dimensional, stratified, isothermal, idealmagnetohydrodynamic shearing box simulations. The strength of theself-sustained toroidal magnetic field depends on the net vertical magneticflux, which we vary across almost the entire range over which the MRI islinearly unstable. We quantify disc structure and dynamo properties as afunction of the initial ratio of mid-plane gas pressure to vertical magneticfield pressure, $\beta_0^{\rm mid} = p_{\rm gas} / p_B$. For $10^5 \geq\beta_0^{\rm mid} \geq 10$ the effective $\alpha$-viscosity parameter scales asa power-law. Dynamo activity persists up to and including $\beta_0^{\rm mid} =10^2$, at which point the entire vertical column of the disc is magneticpressure-dominated. Still stronger fields result in a highly inhomogeneous discstructure, with large density fluctuations. We show that the turbulent steadystate $\beta^{\rm mid}$ in our simulations is well-matched by the analyticmodel of Begelman et al. (2015) describing the creation and buoyant escape oftoroidal field, while the vertical structure of the disc can be broadlyreproduced using this model. Finally, we discuss the implications of ourresults for observed properties of X-ray binaries.

Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes