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Title: Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

Abstract

Here, we study the dynamo generation (exponential growth) of large-scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal (x–y) averaging, we also demonstrate the presence of large-scale fields when vertical (y–z) averaging is employed instead. By computing space–time planar averaged fields and power spectra, we find large-scale dynamo action in the early MRI growth phase – a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large-scale fields exponentially before turbulence and high wavenumber fluctuations arise. Thus the large-scale dynamo requires only linear fluctuations but not non-linear turbulence (as defined by mode–mode coupling). Vertical averaging also allows for monitoring the evolution of the large-scale vertical field and we find that a feedback from horizontal low wavenumber MRI modes provides a clue as to why the large-scale vertical field sustains against turbulent diffusion in the non-linear saturation regime. We compute the terms in the mean field equations to identify the individual contributions to large-scale field growth for both types of averaging. The large-scale fields obtained from vertical averaging are found to compare well with globalmore » simulations and quasi-linear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding the large-scale MRI dynamo saturation and turbulence.« less

Authors:
 [1]; ORCiD logo [1];  [2]
  1. Princeton Univ., Princeton, NJ (United States)
  2. Univ. of Rochester, Rochester, NY (United States)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1332518
Grant/Contract Number:  
SC0012467; HST-AR-13916.002; AST1515648
Resource Type:
Accepted Manuscript
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Volume: 462; Journal Issue: 1; Journal ID: ISSN 0035-8711
Publisher:
Royal Astronomical Society
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; accretion; accretion discs; dynamo; magnetic fields; MHD; turbulence

Citation Formats

Bhat, Pallavi, Ebrahimi, Fatima, and Blackman, Eric G. Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability. United States: N. p., 2016. Web. doi:10.1093/mnras/stw1619.
Bhat, Pallavi, Ebrahimi, Fatima, & Blackman, Eric G. Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability. United States. doi:10.1093/mnras/stw1619.
Bhat, Pallavi, Ebrahimi, Fatima, and Blackman, Eric G. Wed . "Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability". United States. doi:10.1093/mnras/stw1619. https://www.osti.gov/servlets/purl/1332518.
@article{osti_1332518,
title = {Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability},
author = {Bhat, Pallavi and Ebrahimi, Fatima and Blackman, Eric G.},
abstractNote = {Here, we study the dynamo generation (exponential growth) of large-scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal (x–y) averaging, we also demonstrate the presence of large-scale fields when vertical (y–z) averaging is employed instead. By computing space–time planar averaged fields and power spectra, we find large-scale dynamo action in the early MRI growth phase – a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large-scale fields exponentially before turbulence and high wavenumber fluctuations arise. Thus the large-scale dynamo requires only linear fluctuations but not non-linear turbulence (as defined by mode–mode coupling). Vertical averaging also allows for monitoring the evolution of the large-scale vertical field and we find that a feedback from horizontal low wavenumber MRI modes provides a clue as to why the large-scale vertical field sustains against turbulent diffusion in the non-linear saturation regime. We compute the terms in the mean field equations to identify the individual contributions to large-scale field growth for both types of averaging. The large-scale fields obtained from vertical averaging are found to compare well with global simulations and quasi-linear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding the large-scale MRI dynamo saturation and turbulence.},
doi = {10.1093/mnras/stw1619},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 1,
volume = 462,
place = {United States},
year = {2016},
month = {7}
}

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