Demystifying Flux Eruptions: Magnetic Flux Transport in Magnetically Arrested Disks
Journal Article
Overview
abstract
Abstract; ; Magnetically arrested disks (MADs) are a compelling model for explaining variability in low-luminosity active galactic nuclei (AGN), including horizon-scale outbursts. MADs experience powerful flux eruptions—episodic ejections of magnetic flux from the black hole horizon—that may drive the observed luminosity variations. In this work, we develop and validate a new formalism describing large-scale magnetic field transport in general relativistic magnetohydrodynamic simulations of MADs with geometrical thicknesses of; h; /; R; = 0.1 and; h; /; R; = 0.3. We introduce a net flux transport velocity,; v; Φ; , which accounts for both advective and diffusive processes. We show that MADs maintain a statistical quasi-steady state where advection and diffusion nearly balance. Flux eruptions appear as small deviations from this equilibrium, with; v; Φ; /; V; ; k; ; ≪ 1, where; V; ; k; ; is the local Keplerian velocity. Using this framework, we analytically derive a recurrence timescale for flux eruptions,; t; rec; ≃ 1500; r; ; g; ; /; c; . This timescale closely matches simulation results. The smallness of; v; Φ; explains the long recurrence times of flux eruptions compared to other system timescales. We also take a closer look at the diffusion of the magnetic field by performing the first measurement of turbulent resistivity in MADs. We then estimate the turbulent magnetic Prandtl number,; ; ; ; ; ; ; ; P; ; ; m; ; ; ; ; , defined as the ratio of turbulent viscosity to turbulent resistivity. We find; ; ; ; ; ; ; ; P; ; ; m; ; ; ≃; 3; ; ; , consistent with shearing-box simulations of magnetorotational instability-driven turbulence. While flux eruptions excite large-scale nonaxisymmetric modes and locally enhance turbulent resistivity, magnetic field diffusion is dominated by smaller-scale turbulent motions. These results provide new insight into the nature of AGN variability and the fundamental physics of magnetic field transport.;
