Finite-time Response of Dynamo Mean-field Effects in Magnetorotational Turbulence

by Oliver Gressel (Leibniz-Institut f¨ur Astrophysik Potsdam & Niels Bohr International Academy)

Tuesday 18 Oct 2022, 10:00 → 11:00 Europe/Stockholm

https://stockholmuniversity.zoom.us/j/940229961

Accretion disk turbulence along with its effect on large-scale magnetic fields plays an important role in understanding disk evolution in general, and the launching of astrophysical jets in particular. Motivated by enabling a comprehensive subgrid description for global long-term simulations of accretions disks, we aim to further characterize the transport coefficients emerging in local simulations of magnetorotational disk 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 buildup 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 nonlocal closure relation in space, and incorporated into a suitable mean-field description that we briefly sketch out here, the new framework will allow us to drop the restrictive assumption of scale separation.

https://ui.adsabs.harvard.edu/abs/2022ApJ...928..118G/abstract

Gressel_2022-Oct-18.mp4

ogressel_dynamo.pdf