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Instability mechanism and vortex disruption in magnetohydrodynamic shear flows

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The problem of linear stability and nonlinear evolution of magnetohydrodynamic (MHD) shear flows in two-dimensional, homogeneous, incompressible MHD is revisited. In the equivalent hydrodynamic setting, the instability mechanism is typically interpreted in terms of the Counter-propagating Rossby Wave (CRW) mechanism, and as the instability reaches finite amplitude, the shear flow rolls up into vortices that, in a two-dimensional homogeneous system, is stable and long-lived. The talk will focus on the MHD modifications to the CRW mechanism as well as the potential disruption of such long-lived vortices from MHD feedback. An estimate based on the scaling arguments of Weiss (1966) is performed which predicts that vortices will be disrupted when M^2 Rm = O(1), where M is a measure of the initial field strength and Rm is the magnetic Reynolds number. A vortex disruption parameter based on the Okubo—Weiss criterion is utilised to analyse direct numerical simulations of MHD shear flows, and the analyses lend support to the theoretical estimate.

This talk is part of the DAMTP Astrophysics Seminars series.

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