Comparisons and Connections between Mean Field Dynamo Theory and Accretion Disc Theory

TL;DR

This paper explores the relationship between mean field dynamo theory and accretion disc theory, highlighting recent advances and proposing their integration into a unified framework for understanding astrophysical magnetic fields and accretion processes.

Contribution

It synthesizes recent progress in both theories and argues for their unification to better explain the role of magnetic fields in accretion and astrophysical phenomena.

Findings

MFDT has achieved a dynamical saturation theory consistent with simulations.

Progress in accretion disc theory is emerging but not yet as comprehensive.

Large scale magnetic fields likely influence angular momentum transport in accretion discs.

Abstract

The origin of large scale magnetic fields in astrophysical rotators, and the conversion of gravitational energy into radiation near stars and compact objects via accretion have been subjects of active research for a half century. Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both subjects have benefitted from numerical simulations.However, understanding the key principles and practical modeling of observations warrants testable semi-analytic mean field theories that distill the essential physics. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory exemplify this pursuit. That the latter is a mean field theory is not always made explicit but the combination of turbulence and global symmetry imply such. The more commonly explicit presentation of assumptions in 20th century textbook MFDT has exposed it to arguably more widespread criticism than incurred by 20th century alpha-accretion theory despite complementary weaknesses. In the 21st century however, MFDT has experienced a breakthrough with a dynamical saturation theory that consistently agrees with simulations. Such has not yet occurred in accretion disc theory, though progress is emerging. Ironically however, for accretion engines, MFDT and accretion theory are presently two artificially uncoupled pieces of what should be a single coupled theory. Large scale fields and accretion flows are dynamically intertwined because large scale fields likely play a key role in angular momentum transport. I discuss and synthesize aspects of recent progress in MFDT and accretion disc theory to suggest why the two likely conspire in a unified theory.