Finite-time response of dynamo mean-field effects in magnetorotational turbulence

TL;DR

This paper investigates the finite-time response of dynamo mean-field effects in magnetorotational turbulence, providing insights into transport coefficients and phase lag phenomena crucial for modeling accretion disc evolution.

Contribution

It introduces a time-dependent test-field method to characterize the transport coefficients' frequency response and phase lag in magnetorotational turbulence.

Findings

Fourier spectra of transport coefficients are well approximated by a simple response function.

Significant phase lag observed at intermediate timescales.

Finite-time build-up of the electromotive force (EMF) explained by the response function.

Abstract

Accretion disc turbulence along with its effect on large-scale magnetic fields plays an important role in understanding disc evolution in general, and the launching of astrophysical jets in particular. Motivated by enabling a comprehensive sub-grid description for global long-term simulations of accretions discs, we aim to further characterize the transport coefficients emerging in local simulations of magnetorotational disc turbulence. For the current investigation, we leverage a time-dependent version of the test-field method, which is sensitive to the turbulent electromotive force (EMF) generated as a response to a set of pulsating background fields. We obtain Fourier spectra of the transport coefficients as a function of oscillation frequency. These are well approximated by a simple response function, describing a finite-time build-up of the EMF as a result of a time-variable mean magnetic field. For intermediate timescales (i.e., slightly above the orbital frequency), we observe a significant phase lag of the EMF compared to the causing field. Augmented with our previous result on a non-local closure relation in space, and incorporated into a suitable mean-field description that we briefly sketch out here, the new framework will allow to drop the restrictive assumption of scale separation.