Characterising the Dynamo in a Radiatively Inefficient Accretion Flow

TL;DR

This paper investigates the MRI-driven dynamo in radiatively inefficient accretion flows using mean field theory, revealing a high turbulent pumping coefficient and implications for jet launching near black holes.

Contribution

First to map the poloidal distribution of dynamo coefficients in global accretion flow simulations, highlighting the role of the MRI-driven dynamo in jet formation.

Findings

High turbulent pumping coefficient transports magnetic flux outward.

Dynamo coefficients decrease with radius.

Dynamo operates as an alpha-omega type at high latitudes.

Abstract

We explore the MRI driven dynamo in a radiatively inefficient accretion flow (RIAF) using the mean field dynamo paradigm. Using singular value decomposition (SVD) we obtain the least squares fitting dynamo coefficients $\alpha$ and $\gamma$ by comparing the time series of the turbulent electromotive force and the mean magnetic field. Our study is the first one to show the poloidal distribution of these dynamo coefficients in global accretion flow simulations. Surprisingly, we obtain a high value of the turbulent pumping coefficient $\gamma$ which transports the mean magnetic flux radially outward. This would have implications for the launching of magnetised jets which are produced efficiently in presence a large-scale poloidal magnetic field close to the compact object. We present a scenario of a truncated disc beyond the RIAF where a large scale dynamo-generated poloidal magnetic field can aid jet-launching close to the black hole. Magnitude of all the calculated coefficients decreases with radius. Meridional variations of $\alpha_{\phi \phi}$, responsible for toroidal to poloidal field conversion, is very similar to that found in shearing box simulations using the `test field' (TF) method. By estimating the relative importance of $\alpha$-effect and shear, we conclude that the MRI driven large-scale dynamo, which operates at high latitudes beyond a disc scale height, is essentially of the $\alpha-\Omega$ type.