Partially ionised mixing with collisional and radiative ionisation and recombination

Ben Snow

Hosted by University of Exeter, United Kingdom on May 16, 2024


Turbulence is a fundamental process that drives mixing and energy redistribution across a wide range of astrophysical systems. For warm (T ≈ 10^4K) plasma, the material is partially-ionised, consisting of both ionised and neutral species. The interactions between ionised and neutral species are thought to play a key role in heating (or cooling) of partially-ionised plasmas. Here mixing is studied in a two-fluid partially-ionised plasma undergoing the shear-driven Kelvin-Helmholtz instability to evaluate the thermal processes within the mixing layer. 2D 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 ionisation and recombination are included. In the mixing layer, a complex array of ionisation and recombination processes occur as the cooler layer joins the hotter layer, and vice-versa. In localised 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 ionisation-recombination equilibrium, however the obtained equilibrium is different to the Saha-Boltzmann LTE equilibrium. The dynamic mixing processes may be important in determining the ionisation states, and with that intensities of spectral lines, of observed mixing layers.

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