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CGAFD Seminar: Large-scale shear-driven instabilities in stratified MHD with application to the solar tachocline

Large-scale shear-driven instabilities in stratified MHD with application to the solar tachocline


Event details

Abstract

 In the framework of the solar dynamo, shear flows underpin the self-consistent generation of large-scale magnetic fields, notably in the solar tachocline. The strong horizontal magnetic field in this region is generated by horizontally anisotropic flow which itself is driven by a shear-driven instability. It remains however, poorly understood how conditions in the solar tachocline affect this instability. To address this, we study an idealised model of the small-scale dynamics of the solar tachocline in the magnetohydrodynamic (MHD) framework, exploring the effects of stable stratification and a magnetic field on the stability of a Kolmogorov shear flow. 

 

In this talk, we demonstrate that under the assumption of a strong magnetic field, linear stability analysis and a corresponding reduced model show that stable stratification can have a destabilising effect on the stability of a Kolmogorov flow. Asymptotic analysis of magnetised double-diffusive instabilities shows that stable stratification as well as small magnetic and thermal Prandtl numbers are required for their existence, consistent with observations of the solar tachocline based on helioseismological data.

 

We also consider the limit of a weak field, which is expected to play an important role in the solar dynamo. In this limit, we use weakly nonlinear theory to derive an amplitude equation at the onset of hydrodynamic instability for Kolmogorov flow. Direct simulation of this amplitude equation shows that weak stable stratification and magnetic field effects stabilise the flow in this limit.

Location:

Amory C417