How Magnetic Fields Regulate Cooling and Mixing in Turbulent Radiative Mixing Layers

Abstract

Turbulent radiative mixing layers (TRMLs) are expected wherever hot and cold gas move past one another, including in the solar corona, galactic winds, and cold filaments in galaxy clusters. These environments are often magnetized, but magnetic effects on mixing and cooling remain less well understood than in the hydrodynamic (HD) case. We present magnetohydrodynamic (MHD) simulations of TRMLs at resolutions up to $1024 \times 2048^2$, spanning fields aligned with and transverse to the shear, and polarity-reversing configurations in which oppositely directed fields form current sheets at the interface. Even initially weak hot-phase fields, with $\mathcal{M}_{\rm A,shear}\equiv v_{\rm shear}/v_{\rm A}\sim14$, reduce the mass and enthalpy flux and radiative cooling rate by up to an order of magnitude relative to HD. Magnetic tension weakens turbulent motions, reducing both the diffusion of hot gas into the layer and the folding of the cooling surface. Transverse fields suppress cooling somewhat more strongly than shear-aligned ones, although a transverse field exerts no tension against the initial linear instability. Polarity reversal changes the morphology of the cooling gas without restoring HD-like mixing. The dependence on the Damköhler number is similar to the HD case, but the degree of suppression is dependent on the initial field orientation. The net cooling rate appears resolution-independent in HD but declines with resolution in MHD, and has not converged, so our suppression factors are lower limits. Magnetic fields therefore strongly regulate cooling and mixing in multiphase gas, and quantitative predictions require careful treatment of transport processes.