The Structure of Poloidal Fields Embedded in Thin Disks

TL;DR

This paper develops a comprehensive model of poloidal magnetic fields in thin accretion disks, incorporating turbulent diffusivities and helicity effects, to understand magnetic field structure and angular momentum transport.

Contribution

It introduces a set of equations for stationary disks with embedded poloidal fields, including tensorial turbulent diffusivities and a helicity-regulated dynamo, advancing the understanding of magnetic field structure in thin disks.

Findings

Vertical profiles of diffusion coefficients influence magnetic field geometry.

Large bending angles in poloidal fields can dominate angular momentum flux.

Non-unique solutions arise, requiring additional constraints.

Abstract

Many accreting systems are modeled as geometrically thin disks. Simulations of accretion disks cannot be extended to this regime, although local models can address the behavior of narrow annuli. A global model needs to account for the interactions between a large-scale poloidal field, accreted from the environment, and the disk. The disk magnetosphere can be modeled subject to the boundary conditions imposed by the disk. These depend on the structure of the magnetic field as it crosses the disk and the degree to which the disk can support a bend in the field lines. Building on earlier work we derive a set of equations describing a stationary disk with an embedded poloidal field. We derive a modified induction equation that incorporates tensorial turbulent diffusivities and a helicity-regulated $\alpha$-effect. We quantify how helicity conservation introduces a nonlinear backreaction on the large-scale dynamo, dynamically coupling turbulent diffusion and $\alpha$-quenching. We discuss the challenges encountered in finding a unique solution under stationary flows $E_\phi =0$, which balances the inflow of $B_z$ due to accretion, the outflow due to radial diffusion of $B_z$, and the vertical movement of $B_r$ due to turbulent diffusion and buoyancy. The vertical profiles of both the azimuthal diffusion coefficient $D_{ijk}$ and the helicity-driven $\alpha_{ij}$ demonstrate that changes in the radial gradient can restructure the magnetic field geometry. The ability of disks to sustain large bending angles in the poloidal field implies that angular momentum flux through the magnetosphere can dominate over internal transport even for weak fields. Competing factors can result in non-unique solutions, necessitating extra constraints and diagnostics that highlight the role of isotropic turbulence and helicity regulation in magnetized disk environments.