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Collisional ionisation, recombination and ionisation potential in two-fluid slow-mode shocks: analytical and numerical results

Shocks are a universal feature of the lower solar atmosphere which consists of both ionised and neutral species. Including partial ionisation leads to a finite-width existing for shocks, where the ionised and neutral species decouple and recouple. As such, drift velocities exist within the shock that lead to frictional heating between the two species, in addition to the adiabatic temperature changes across the shock. The local temperature enhancements within the shock alter the recombination and ionisation rates and hence change the composition of the plasma. We study the role of collisional ionisation and recombination in slow-mode partially-ionised shocks. In particular we incorporate the ionisation potential energy loss and analyse the consequences of having a non-conservative energy equation. A semi-analytical approach is used to determine the possible equilibrium shock jumps for a two-fluid model with ionisation, recombination, ionisation potential and arbitrary heating. Two-fluid numerical simulations are performed using the (P\underline{I}P) code. Results are compared to the MHD model and semi-analytic solution. Accounting for ionisation, recombination and ionisation potential significantly alters the behaviour of shocks in both substructure and post-shock regions. In particular, for a given temperature, equilibrium can only exist for specific densities due to the radiative losses needing to be balanced by the heating function. A consequence of the ionisation potential is that a compressional shock will lead to a reduction of temperature in the post-shock region, rather than the increase seen for MHD. The numerical simulations pair well with the derived analytic model for shock velocities.