On the existence of shear-current effects in magnetized burgulence. (arXiv:2006.05661v3 [physics.flu-dyn] UPDATED)

<a href="http://arxiv.org/find/physics/1/au:+Kapyla_M/0/1/0/all/0/1">Maarit J. Käpylä</a>, <a href="http://arxiv.org/find/physics/1/au:+Vizoso_J/0/1/0/all/0/1">Javier Álvarez Vizoso</a>, <a href="http://arxiv.org/find/physics/1/au:+Rheinhardt_M/0/1/0/all/0/1">Matthias Rheinhardt</a>, <a href="http://arxiv.org/find/physics/1/au:+Brandenburg_A/0/1/0/all/0/1">Axel Brandenburg</a>, <a href="http://arxiv.org/find/physics/1/au:+Singh_N/0/1/0/all/0/1">Nishant K. Singh</a>

The possibility of explaining shear flow dynamos by a magnetic shear-current

(MSC) effect is examined via numerical simulations. Our primary diagnostics is

the determination of the turbulent magnetic diffusivity tensor

$boldsymbol{eta}$. In our setup, a negative sign of its component $eta_{yx}$

is necessary for coherent dynamo action by the SC effect. To be able to measure

turbulent transport coefficients from systems with magnetic background

turbulence, we present an extension of the test-field method (TFM), applicable

to our setup where the pressure gradient is dropped from the momentum equation:

the nonlinear TFM (NLTFM). Our momentum equation is related to Burgers’

equation and the resulting flows are referred to as magnetized burgulence. We

use both stochastic kinetic and magnetic forcings to mimic cases without and

with simultaneous small-scale dynamo action (SSD). When we force only

kinetically, negative $eta_{yx}$ are obtained with exponential growth in both

the radial and azimuthal mean magnetic field components. Using isotropic

magnetic forcing, the field growth is no longer exponential, while NLTFM yields

positive $eta_{yx}$. By employing an alternative forcing from which

wavevectors having small components are removed, the exponential growth is

recovered, but the NLTFM results do not change significantly. Analyzing the

dynamo excitation conditions for the coherent SC and incoherent $alpha$ and SC

effects shows that the incoherent effects are the main drivers of the dynamo in

the majority of cases. We find no evidence for MSC-effect-driven dynamos in our

simulations.

The possibility of explaining shear flow dynamos by a magnetic shear-current

(MSC) effect is examined via numerical simulations. Our primary diagnostics is

the determination of the turbulent magnetic diffusivity tensor

$boldsymbol{eta}$. In our setup, a negative sign of its component $eta_{yx}$

is necessary for coherent dynamo action by the SC effect. To be able to measure

turbulent transport coefficients from systems with magnetic background

turbulence, we present an extension of the test-field method (TFM), applicable

to our setup where the pressure gradient is dropped from the momentum equation:

the nonlinear TFM (NLTFM). Our momentum equation is related to Burgers’

equation and the resulting flows are referred to as magnetized burgulence. We

use both stochastic kinetic and magnetic forcings to mimic cases without and

with simultaneous small-scale dynamo action (SSD). When we force only

kinetically, negative $eta_{yx}$ are obtained with exponential growth in both

the radial and azimuthal mean magnetic field components. Using isotropic

magnetic forcing, the field growth is no longer exponential, while NLTFM yields

positive $eta_{yx}$. By employing an alternative forcing from which

wavevectors having small components are removed, the exponential growth is

recovered, but the NLTFM results do not change significantly. Analyzing the

dynamo excitation conditions for the coherent SC and incoherent $alpha$ and SC

effects shows that the incoherent effects are the main drivers of the dynamo in

the majority of cases. We find no evidence for MSC-effect-driven dynamos in our

simulations.

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