Strongly magnetized accretion discs: structure and accretion from global magnetohydrodynamic simulations

Abstract

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 disks 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 disks' vertical structure in all three cases, with accretion occurring at z/R ≈ 0.2 in the two most strongly magnetized models. The disk midplane 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 β 0 to those seen in shearing box simulations, though the disk midplane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disk 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 disk. We suggest that geometrically thin accretion disks 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 disk phenomenology in X-ray binaries (XRBs) and active galactic nuclei (AGN).

Magnetic field transport in compact binaries

“…These simulations typically start with a torus and allow the accretion disk to form. However, the most recent MHD simulations, which initially start with an accretion disk with a constant β, tend to show that the magnetic flux is neither advecting inwards or outwards (Zhu & Stone 2018;Mishra et al 2020), which is at odds with the GRMHD results. This result, surprisingly, seems to hold whatever value of β between 10 2 and 10 4 .…”

“…We note that the prescriptions from Jacquemin-Ide et al 2019are selfsimilar and so, by nature, they take into account the global structure of the disk. However, due to the self-similar approximation, the solutions from Jacquemin-Ide et al 2019have a uniform magnetization (not dissimilar to global simulations from Zhu & Stone 2018;Mishra et al 2020) and are not consistent with a realistic disk where the magnetization depends on the radius and is not self-similar. This is especially true close to the fronts.…”

“…Taking into account these high magnetization instabilities would require a detailed study providing a simple prescription of the mass, angular momentum (see Marshall et al 2018), energy, and magnetic field transport that we can use in our model. However, it is not clear that a simple axisymetric model would be able to reproduce the long-term behavior of high-magnetization simulations, which are reported to be highly non-axisymetric with spiral patterns that are possibly related to the properties of transport (McKinney et al 2012;Mishra et al 2020). Beckwith et al (2009) also expressed concerns regarding the use of a mean field approach to treat the turbulence.…”

Magnetic outflows from turbulent accretion disks

“…In these models, accretion typically occurs in the disk bulk and angular momentum escapes through a magnetised outflow launched from the disk surface (Murphy et al 2010;Stepanovs & Fendt 2016). Even though some effective models predict accretion preferentially located within the disk surface ( 3h: Guilet & Ogilvie 2013;Jacquemin-Ide et al 2019), where h is the disk pressure scale-height, none of these models produce accretion in the disk atmosphere at z ∼ 10 h, as is observed in the numerical simulations of Zhu & Stone (2018) and Mishra et al (2020). Furthermore, Zhu & Stone (2018; see their Fig.…”

“…To this end, global numerical simulations combining MRI-driven turbulence and magnetically-driven outflows have been computed (Suzuki & Inutsuka 2014;Zhu & Stone 2018;Mishra et al 2020). These simulations are known to exhibit an exotic global configuration.…”