Radially dependent large-scale dynamos in global cylindrical shear flows and the local cartesian limit

2017

Phys. Rev. E

From numerical simulations, we show that non-rotating magnetohydrodynamic shear flows are unstable to finite amplitude velocity perturbations and become turbulent, leading to the growth and sustenance of magnetic energy, including large scale fields. This supports the concept that sustained magnetic energy from turbulence is independent of the driving mechanism for large enough magnetic Reynolds numbers.

Section: Discussionmentioning

confidence: 99%

Sustained turbulence and magnetic energy in nonrotating shear flows

Nauman

2017

Phys. Rev. E

“…In contrast to these studies using mode-coupling/turbulence, Ebrahimi & Blackman (2016) (EB16 from here on) show from a single mode quasilinear analysis, it is possible to obtain an EMF required for large scale dynamo action, with the minimum requirement being that the perturbations need to be of non-axisymmetric nature. This study did not focus on the secular cycle periods but on the initial exponential generation of vertically averaged mean fields which emerges on the MRI growth time scale directly as a global mode for the size of the system under study.…”

Section: Introductionmentioning

confidence: 96%

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

Bhat

Ebrahimi

Mon. Not. R. Astron. Soc.

2016

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 demonstrate the presence of large scale fields when either horizontal or vertical (y-z) averaging is employed. By computing planar averaged fields and power spectra, we find large scale dynamo action in the early MRI growth phase-a previously unidentified feature. Fast growing horizontal low modes and fiducial vertical modes over a narrow range of wave numbers amplify these planar averaged fields in the MRI growth phase, before turbulence sets in. The large scale field growth requires linear fluctuations but not nonlinear turbulence (as defined by mode-mode coupling) and grows as a direct global mode of the MRI. Only by vertical averaging, can it be shown that the growth of horizontal low wavenumber MRI modes directly feed-back to the initial vertical field providing a clue as to why the large scale vertical field sustains against turbulent diffusion in the saturation regime. We compute the terms in the planar averaged mean field equations to identify the individual contributions to large scale field growth for both vertical and horizontal averaging. The large scale fields obtained from such vertical averaging are found to compare well with global cylindrical simulations and quasilinear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding large scale MRI dynamo saturation and turbulence.

“…We find that the peak at k = 1 appears and disappears periodically, consistent with temporal cycles in the large-scale dynamo associated with planar averaged fields. The minimum requirement for large-scale MRI dynamo growth has been shown to be anistropic fluctuations and shear to form a nonzero EMF (Ebrahimi & Blackman 2016). The form of EMF in terms of a mean field theory, such as incoherent alphashear or helicity flux source, is yet to be investigated (Vishniac & Brandenburg 1997;Vishniac & Cho 2001;Ebrahimi & Bhattacharjee 2014).…”

Section: Spectral Analysis Of Mri Growthmentioning

confidence: 99%

Evolution of the magnetorotational instability on initially tangled magnetic fields

Bhat

Ebrahimi

Monthly Notices of the Royal Astronomical Society

et al. 2017

The initial magnetic field of previous magnetorotational instability (MRI) simulations has always included a significant system-scale component, even if stochastic. However, it is of conceptual and practical interest to assess whether the MRI can grow when the initial field is turbulent. The ubiquitous presence of turbulent or random flows in astrophysical plasmas generically leads to a small-scale dynamo (SSD), which would provide initial seed turbulent velocity and magnetic fields in the plasma that becomes an accretion disc. Can the MRI grow from these more realistic initial conditions? To address this we supply a standard shearing box with isotropically forced SSD generated magnetic and velocity fields as initial conditions, and remove the forcing. We find that if the initially supplied fields are too weak or too incoherent, they decay from the initial turbulent cascade faster than they can grow via the MRI. When the initially supplied fields are sufficient to allow MRI growth and sustenance, the saturated stresses, large-scale fields, and power spectra match those of the standard zero net flux MRI simulation with an initial large scale vertical field.