The standard magnetorotational instability (SMRI) with a magnetic field component parallel to the rotation axis is widely believed to be responsible for the fast accretion in astronomical disks. In conventional base flows with a Keplerian profile or an ideal Couette profile, most studies focus on axisymmetric SMRI, since excitation of nonaxisymmetric SMRI in such flows requires a magnetic Reynolds number (Rm) more than an order of magnitude larger. Here, we report that, in a magnetized Taylor-Couette flow, nonaxisymmetric SMRI with an azimuthal mode number m = 1 can be triggered by a free-shear layer in the base flow at Rm ≳1, the same threshold as for axisymmetric SMRI. Global linear analysis reveals that the free-shear layer reduces the required Rm, possibly by introducing an extremum in the vorticity of the base flow. Nonlinear simulations validate the results from linear analysis and confirm that a novel instability recently discovered experimentally [Wang et al., Nat. Commun. 13, 4679 (2022)] is the nonaxisymmetric m =1 SMRI. Further, our finding has astronomical implications as free-shear layers are ubiquitous in celestial systems, such as the disk-star boundary layer, the solar tachocline, and the edge of planet-opened gaps in protoplanetary disks.
Wang, Yin, et al. "Observation of Nonaxisymmetric Standard Magnetorotational Instability Induced by a Free-Shear Layer." Physical Review Letters, vol. 134, no. 13, Mar. 2025. https://doi.org/10.1103/physrevlett.134.135101
@article{osti_2547002,
author = {Wang, Yin and Ebrahimi, Fatima and Lu, Hongke and Goodman, Jeremy and Gilson, Erik P. and Ji, Hantao},
title = {Observation of Nonaxisymmetric Standard Magnetorotational Instability Induced by a Free-Shear Layer},
annote = {The standard magnetorotational instability (SMRI) with a magnetic field component parallel to the rotation axis is widely believed to be responsible for the fast accretion in astronomical disks. In conventional base flows with a Keplerian profile or an ideal Couette profile, most studies focus on axisymmetric SMRI, since excitation of nonaxisymmetric SMRI in such flows requires a magnetic Reynolds number (Rm) more than an order of magnitude larger. Here, we report that, in a magnetized Taylor-Couette flow, nonaxisymmetric SMRI with an azimuthal mode number m = 1 can be triggered by a free-shear layer in the base flow at Rm ≳1, the same threshold as for axisymmetric SMRI. Global linear analysis reveals that the free-shear layer reduces the required Rm, possibly by introducing an extremum in the vorticity of the base flow. Nonlinear simulations validate the results from linear analysis and confirm that a novel instability recently discovered experimentally [Wang et al., Nat. Commun. 13, 4679 (2022)] is the nonaxisymmetric m =1 SMRI. Further, our finding has astronomical implications as free-shear layers are ubiquitous in celestial systems, such as the disk-star boundary layer, the solar tachocline, and the edge of planet-opened gaps in protoplanetary disks.},
doi = {10.1103/physrevlett.134.135101},
url = {https://www.osti.gov/biblio/2547002},
journal = {Physical Review Letters},
issn = {ISSN 0031-9007},
number = {13},
volume = {134},
place = {United States},
publisher = {American Physical Society (APS)},
year = {2025},
month = {03}}
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