Helical and nonhelical large-scale dynamos in thin accretion discs

TL;DR

This paper develops a coupled disc and local simulation model to study large-scale magnetic dynamos in thin accretion discs, revealing detailed field structures, dynamo cycles, and exploring non-helical dynamo mechanisms.

Contribution

It introduces a new method combining thin-disc models with local simulations for more realistic dynamo modeling, including non-helical mechanisms.

Findings

Saturated magnetic fields have dipole configurations with plasma β between 0.1 and 100.

Dynamo cycle period is approximately 40 orbital timescales.

Non-helical dynamos can operate in thin discs but produce different field configurations.

Abstract

The dynamics of accreting and outgoing flows around compact objects depends crucially on the strengths and configurations of the magnetic fields therein, especially of the large-scale fields that remain coherent beyond turbulence scales. Possible origins of these large-scale magnetic fields include flux advection and disc dynamo actions. However, most numerical simulations have to adopt an initially strong large-scale field rather than allow them to be self-consistently advected or amplified, due to limited computational resources. The situation can be partially cured by using sub-grid models where dynamo actions only reachable at high resolutions are mimicked by artificial terms in low-resolution simulations. In this work, we couple thin-disc models with local shearing-box simulation results to facilitate more realistic sub-grid dynamo implementations. For helical dynamos, detailed spatial profiles of dynamo drivers inferred from local simulations are used, and the nonlinear quenching and saturation is constrained by magnetic helicity evolution. In the inner disc region, saturated fields have dipole configurations and the plasma $\beta$ reaches $\simeq 0.1$ to $100$, with correlation lengths $\simeq h$ in the vertical direction and $\simeq 10h$ in the radial direction, where $h$ is the disc scale height. The dynamo cycle period is $\simeq 40$ orbital time scale, compatible with previous global simulations. Additionally, we explore two dynamo mechanisms which do not require a net kinetic helicity and have only been studied in shearing-box setups. We show that such dynamos are possible in thin accretion discs, but produce field configurations that are incompatible with previous results. We discuss implications for future general-relativistic magnetohydrodynamics simulations.