Rayleigh–Taylor instability in partially ionised plasmas: two-fluid linear theory and nonlinear ambipolar diffusion effects

Abstract: The Rayleigh–Taylor instability (RTI) is a key mechanism driving mixing and structure formation in magnetised astrophysical plasmas. In many environments of the interstellar medium the gas is only partially ionised, so that ions and neutrals can drift relative to each other and exchange momentum through collisions. The linear behaviour of the magnetised RTI in this regime has remained poorly characterised.In this talk I present a two-fluid study of the magnetised RTI in partially ionised plasmas. I first derive the linear dispersion relation including ion–neutral collisional coupling. The analysis shows how the growth rate depends on the collision frequency, ionisation fraction and magnetic field orientation, and recovers the ideal MHD and hydrodynamic limits.I then compare these predictions with high-resolution two-fluid simulations performed with the MPI-AMRVAC code. The numerical results reproduce the theoretical growth rates over a wide range of coupling regimes.Finally, I discuss the nonlinear evolution of the instability. The simulations show that ambipolar diffusion redistributes part of the kinetic energy into ion–neutral drift and modifies the morphology of the mixing layer, producing smoother interfaces and reduced small-scale structure compared to ideal MHD.