Strongly magnetized accretion discs: structure and accretion from global magnetohydrodynamic simulations
We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin (H/r ≈ 0.05) accretion discs in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios β _0 = 1000, 300, and 100, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the discs' vertical structure in all three cases, with accretion occurring at z/R ≈ 0.2 in the two most strongly magnetized models. The disc mid-plane shows outflow rather than accretion. Accretion through the surface layers is driven mainly by stress due to coherent large-scale magnetic field rather than by turbulent stress. Equivalent viscosity parameters measured from our simulations show similar dependencies on initial β0 to those seen in shearing box simulations, though the disc mid-plane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disc evolution. Over the (limited) duration of our simulations, we find evidence that the net flux attains a quasi-steady state at levels that can stably maintain a strongly magnetized disc. We suggest that geometrically thin accretion discs in observed systems may commonly exist in a magnetically `elevated' state, characterized by non-zero but modest vertical magnetic fluxes, with potentially important implications for disc phenomenology in X-ray binaries and active galactic nuclei.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- February 2020
- accretion discs;
- galaxies: magnetic fields;
- Astrophysics - High Energy Astrophysical Phenomena