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Viscoresistive magnetohydrodynamic turbulence, driven by a twodimensional unstable shear layer that is maintained by an imposed body force, is examined by decomposing it into dissipationless linear eigenmodes of the initial profiles. The downgradient momentum flux, as expected, originates from the largescale instability. However, continual upgradient momentum transport by largescale linearly stable but nonlinearly excited eigenmodes is identified, and found to nearly cancel the downgradient transport by unstable modes. The stable modes effectuate this by depleting the largescale turbulent fluctuations via energy transfer to the mean flow. This establishes a physical mechanism underlying the longknown observation that coherent vortices formed from nonlinear saturation of the instability reduce turbulent transport and fluctuations, as such vortices are composed of both the stable and unstable modes, which are nearly equal in their amplitudes. The impact of magnetic fields on the nonlinearly excited stable modes is then quantified. Even when imposing a strong magnetic field that almost completely suppresses the instability, the upgradient transport by the stable modes is at least twothirds of the downgradient transport by the unstable modes, whereas for weaker fields, this fraction reaches up to 98% . These effects are persistent with variations in magnetic Prandtl number and forcing strength. Finally, continuum modes are shown to be energetically less important, but essential for capturing the magnetic fluctuations and Maxwell stress. A simple analytical scaling law is derived for their saturated turbulent amplitudes. It predicts the falloff rate as the inverse of the Fourier wavenumber, a property which is confirmed in numerical simulations.more » « less

Straining of magnetic fields by largescale shear flow, which is generally assumed to lead to intensification and generation of small scales, is reexamined in light of the persistent observation of largescale magnetic fields in astrophysics. It is shown that, in magnetohydrodynamic turbulence, unstable shear flows have the unexpected effect of sequestering magnetic energy at large scales due to counteracting straining motion of nonlinearly excited largescale stable eigenmodes. This effect is quantified via dissipation rates, energy transfer rates, and visualizations of magnetic field evolution by artificially removing the stable modes. These analyses show that predictions based upon physics of the linear instability alone miss substantial dynamics, including those of magnetic fluctuations.more » « less

Viscoresistive magnetohydrodynamic turbulence, driven by a twodimensional unstable shear layer that is maintained by an imposed body force, is examined by decomposing it into dissipationless linear eigenmodes of the initial profiles. The downgradient momentum flux, as expected, originates from the largescale instability. However, continual upgradient momentum transport by largescale linearly stable but nonlinearly excited eigenmodes is identified and found to nearly cancel the downgradient transport by unstable modes. The stable modes effectuate this by depleting the largescale turbulent fluctuations via energy transfer to the mean flow. This establishes a physical mechanism underlying the longknown observation that coherent vortices formed from nonlinear saturation of the instability reduce turbulent transport and fluctuations, as such vortices are composed of both the stable and unstable modes, which are nearly equal in their amplitudes. The impact of magnetic fields on the nonlinearly excited stable modes is then quantified. Even when imposing a strong magnetic field that almost completely suppresses the instability, the upgradient transport by the stable modes is at least twothirds of the downgradient transport by the unstable modes, whereas for weaker fields, this fraction reaches up to 98%. These effects are persistent with variations in magnetic Prandtl number and forcing strength. Finally, continuum modes are shown to be energetically less important, but essential for capturing the magnetic fluctuations and Maxwell stress. A simple analytical scaling law is derived for their saturated turbulent amplitudes. It predicts the falloff rate as the inverse of the Fourier wavenumber, a property which is confirmed in numerical simulations.