Eigenvalue bounds for compressible stratified magneto-shear flows varying in two transverse directions

TL;DR

This paper derives three eigenvalue bounds for the instability of ideal compressible stratified magnetohydrodynamic shear flows varying in two transverse directions, extending classical stability theories and revealing conditions for stability.

Contribution

It introduces three novel eigenvalue bounds for stratified MHD shear flows with two-directional variation, generalizing existing stability theories and identifying magnetic field effects.

Findings

The first bound combines the Howard semi-circle theorem with the energy principle, requiring no special conditions.

The second and third bounds generalize Miles-Howard theory and relate to semi-ellipse theorems.

The Miles-Howard stability condition applies only when magnetic field is absent and shear and stratification are aligned.

Abstract

Three eigenvalue bounds are derived for the instability of ideal compressible stratified magnetohydrodynamic shear flows in which the base velocity, density, and magnetic field vary in two directions. The first bound can be obtained by combining the Howard semi-circle theorem with the energy principle of the Lagrangian displacement. Remarkably, no special conditions are needed to use this bound, and for some cases, we can establish the stability of the flow. The second and third bounds come out from a generalisation of the Miles-Howard theory and have some similarity to the semi-ellipse theorem by Kochar & Jain (J. Fluid Mech., vol. 91, 1979, 489) and the bound found by Cally (Astrophys. Fluid Dyn., vol. 31,1983, 43), respectively. An important byproduct of this investigation is that the Miles-Howard stability condition holds only when there is no applied magnetic field and, in addition, the directions of the shear and the stratification are aligned everywhere.