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Shedding light on the MRI-driven dynamo in a stratified shearing box.

Authors :
Dhang, Prasun
Bendre, Abhijit B
Subramanian, Kandaswamy
Source :
Monthly Notices of the Royal Astronomical Society; May2024, Vol. 530 Issue 3, p2778-2794, 17p
Publication Year :
2024

Abstract

We study the magneto-rotational instability (MRI) dynamo in a geometrically thin disc (H / R ≪ 1) using stratified zero net (vertical) flux shearing box simulations. We find that mean fields and electromotive forces (EMFs) oscillate with a primary frequency f <subscript>dyn</subscript> = 0.017 Ω (approximately nine orbital period), but also have higher harmonics at 3 f <subscript>dyn</subscript>. Correspondingly, the current helicity has two frequencies 2 f <subscript>dyn</subscript> and 4 f <subscript>dyn</subscript>, which appear to be the beat frequencies of mean fields and EMFs, respectively, as expected from the magnetic helicity density evolution equation. Further, we adopt a novel inversion algorithm called the 'Iterative Removal Of Sources', to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and EMFs obtained from the shearing box simulation. We show that an α-effect (α<subscript> yy </subscript>) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field, indicating that an α–Ω-type dynamo is operative in MRI-driven accretion discs. We also find that both strong outflow (⁠|$\bar{v}_z$|⁠) and turbulent pumping (γ<subscript> z </subscript>) transport mean fields away from the mid-plane. Instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. We find encouraging evidence that a generative helicity flux is responsible for the effective α-effect. Finally, we point out potential limitations of horizontal (x − y) averaging in defining the 'mean' on the extraction of dynamo coefficients and their physical interpretations. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
00358711
Volume :
530
Issue :
3
Database :
Complementary Index
Journal :
Monthly Notices of the Royal Astronomical Society
Publication Type :
Academic Journal
Accession number :
177106866
Full Text :
https://doi.org/10.1093/mnras/stae1011