Nonmodal growth of MHD shear flows with stabilizing magnetic fields

Shear flows are common and important in astrophysical plasmas, where they often serve as free energy sources able to drive fluctuations and turbulence. In turn, these motions enhance momentum, energy, and particle transport, thus crucially affecting the evolution of the system. The parameter boundaries delineating shear-driven fluctuations are often assumed to be provided by normal-mode linear stability analyses. However, in many important systems, including pipe flow and stratified shear flows in neutral fluids, such analyses are known to be misleading. Perturbations can grow significantly through nonmodal or non-normal growth, potentially driving turbulence and mixing even at parameters where linear stability analyses predict no growth. Here, we explore the degree of nonmodal growth when equilibrium magnetic fields aligned with the flow nearly or entirely stabilize shear-flow instabilities. Pseudospectra indicate significant nonmodal amplification in linearly stable regimes for 2D systems. We also present progress towards comparisons of 2D and 3D optimal linear perturbations that maximize fluctuation growth.