Origin of net vertical magnetic flux in AGN accretion disks

Determine the physical mechanisms that establish and maintain a net vertical magnetic field (net poloidal magnetic flux) threading active galactic nucleus accretion disks.

Background

To investigate whether magnetic pressure can stabilize AGN disks against gravitational instability, the study initializes simulations with a net vertical magnetic field. Prior numerical work suggests that when the midplane magnetization is sufficiently strong (β_mid0 ≲ 103), magnetic pressure can dominate gas pressure and substantially alter disk dynamics, including enhanced angular momentum transport and potential wind launching.

Despite the theoretical and numerical motivation for considering net vertical flux, the astrophysical process by which a net poloidal field would be generated or sustained in AGN disks is not established. Clarifying this mechanism is important for assessing how commonly real AGN disks can achieve the magnetically dominated states that the simulations show can suppress gravitational fragmentation.

References

While net flux is not required for the MRI to provide effective α-viscosity, and the means by which a net vertical field is established are still unknown, there are several reasons we opt for net vertical flux in our simulations.

— Magnetic Pressure Dominance Stabilizes AGN Disks Against Gravitational Instability  (2508.16842 - Gerling-Dunsmore et al., 22 Aug 2025) in Section 2.2 (Simulation Setup)

The thin-disc value of $v_\psi/\eta_{\mathrm{eff}}$ is a long-standing open problem \citep{lubow1994,guilet2012,guilet2014,okuzumi2014,baistone2017,leung2019}, so we treat $\mathcal{R}_m$ (equivalently $\tau_B$) as the controlling unknown that brackets the two outcomes, a target for inference rather than a prediction.

— Bridging magnetothermal winds and photoevaporation to model discs dispersal  (2609.18334 - Picogna et al., 16 Sep 2026) in Section 5.2, “The poloidal induction equation and the limits of a 1D solution”

They trace this to the sign of $J_\phi B_z$, and leave open whether the convex curvature is physical or an artefact of an initial condition with vanishing $B_\phi$.

— Bridging magnetothermal winds and photoevaporation to model discs dispersal  (2609.18334 - Picogna et al., 16 Sep 2026) in Appendix, Section “A first test of the poloidal flux-transport closure”