Virtual Nordic Dynamo Seminar

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

Name: A New Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow Turbulence
Start: 2023-01-31T15:00:00+01:00
End: 2023-01-31T16:00:00+01:00
Location: No location set

by Bindesh Tripathi (University of Wisconsin-Madison)

Tuesday 31 Jan 2023, 15:00 → 16:00 Europe/Stockholm

Description

https://stockholmuniversity.zoom.us/j/940229961

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]. 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]. Computations of energy transfer between the non-orthogonal eigenmodes reveal the channels via which stable modes nonlinearly receive energy. Near-identical impact of stable modes is found in 3D turbulence, as well [3]. It is suggested that the nonlinearly excited stable modes may also play an important role in shear-driven dynamos and reconnection-driven outflows, which can be consequential to the emerging theories of the MHD turbulence.

[1] B. Tripathi, A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel, Phys. Plasmas 29, 070701 (2022). DOI.org/10.1063/5.0096886

[2] B. Tripathi, A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel, Phys. Plasmas 29, 092301 (2022). DOI.org/10.1063/5.0101434

[3] B. Tripathi, A.E. Fraser, P.W. Terry, E.G. Zweibel, and M.J. Pueschel, in prep.