Strongly magnetized accretion discs require poloidal flux

TL;DR

This paper investigates the conditions under which strongly magnetized accretion discs around black holes can develop and persist, emphasizing the necessity of poloidal flux for their stability.

Contribution

It demonstrates through simulations that strong toroidal fields cannot be sustained without net vertical magnetic flux, highlighting the importance of poloidal flux in magnetized discs.

Findings

Discs with zero net vertical flux cannot sustain strong toroidal fields.

Strong magnetization requires initial net vertical magnetic flux.

Poloidal flux is essential for stable, strongly magnetized accretion discs.

Abstract

Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.