Kelvin–Helmholtz-induced mixing in multi-fluid partially ionized plasmas

Abstract

Turbulence is a fundamental process that drives mixing and energy redistribution across a wide range of astrophysical systems. For warm ( T ≈ 10 4 K ) plasma, the material is partially ionized, consisting of both ionized and neutral species. The interactions between ionized and neutral species are thought to play a key role in heating (or cooling) of partially ionized plasmas. Here, mixing is studied in a two-fluid partially ionized plasma undergoing the shear-driven Kelvin–Helmholtz instability to evaluate the thermal processes within the mixing layer. Two-dimensional numerical simulations are performed using the open-source (PIP) code that solves for a two-fluid plasma consisting of a charge-neutral plasma and multiple excited states of neutral hydrogen. Both collisional and radiative ionization and recombination are included. In the mixing layer, a complex array of ionization and recombination processes occur as the cooler layer joins the hotter layer, and vice versa. In localized areas of the mixing layer, the temperature exceeds the initial temperatures of either layer with heating dominated by collisional recombinations over turbulent dissipation. The mixing layer is in approximate ionization-recombination equilibrium, however the obtained equilibrium is different to the Saha–Boltzmann local thermal equilibrium. The dynamic mixing processes may be important in determining the ionization states, and with that intensities of spectral lines, of observed mixing layers. This article is part of the theme issue ‘Partially ionized plasma of the solar atmosphere: recent advances and future pathways’.

Partially ionized plasma of the solar atmosphere: recent advances and future pathways

“…Another key aspect of instabilities is the ability to mix different material together, with the neutrals in partially ionized plasma able to act as a transport mechanism for mass and thermal energy. Snow & Hillier [ 10 ] looked at the role of the Kelvin–Helmholtz instability and Lukin et al [ 11 ] looked at the Rayleigh–Taylor instability to mix prominence plasma. Both studies highlighted different dynamical processes that could drive ionization and recombination and with that complex thermodynamic evolution.…”