A New Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow Turbulence

Straining of magnetic fields by shear flow is widely recognized to generate small-scale magnetohydrodynamic (MHD) turbulence. In astrophysics, where shear flows are ubiquitous, this is at odds with observations of large-scale magnetic fields in stars, galaxies, and beyond. Identified here is a previously unknown mechanism that sequesters magnetic energy at large scales when the flows are unstable.

Kelvin-Helmholtz instability excites, through nonlinear effects, large-scale linearly stable eigenmodes, including an important conjugate-root of the instability. It is shown, by removing these latter stable (damped) modes in high resolution numerical simulations of two-dimensional (2D) MHD turbulence, that the flow composed of the unstable modes alone distorts and folds the magnetic field lines unimpeded, rapidly producing a small-scale energy cascade. Retaining the stable modes, in contrast, significantly weakens the small-scale dissipation, and the rapid generation of small scales is not observed.¹ Computations of energy transfer between the eigenmodes reveal appreciable cancellation of up- and down-scale kinetic and magnetic cascades. In the shear layer, up-gradient momentum transport by the stable modes is found to cancel 70%-90% of down-gradient momentum transport by the unstable modes. These findings persist even with a strong flow-aligned magnetic field that nearly quenches the instability, and with variations in the magnetic Prandtl number from 0.1 to 10.² Near-identical impact of stable modes is found in 3D turbulence, as well. It is suggested that the nonlinearly excited stable modes may also play an important role in reconnection-driven outflows, which can be consequential to the emerging theories of the MHD turbulence.

[1] Tripathi et al., Phys. Plasmas 29, 070701 (2022). DOI:10.1063/5.0096886

[2] Tripathi et al., submitted to PoP.