Non-equilibrium cooling behaviour of static coronal condensation models

The lack of opacity effects and radiative equilibrium are often invoked as limitations of coronal condensation models at low temperatures, causing overcooling relative to the full solution. Previous radiative equilibrium studies of these structures, including radiative transfer outside of local thermodynamic equilibrium, have only considered the instantaneous statistical equilibrium solution, i.e. moving directly between steady-state atomic ionisation and excitation states. Here, we show the importance of considering non-equilibrium ionisation and recombination effects during the radiative relaxation of these models, by performing a set of simulations in which initially isothermal and isobaric slabs cool (without allowing for bulk velocities). We find that due to the ionisation timescale, with a time-dependent treatment, a slab which starts with a high ionisation fraction, rapidly drops to low temperatures (<2kK), before slowly heating back up radiatively to the same temperature as a model using statistical equilibrium, over the course of a few hundred seconds. We discuss potential implications for cooling plasma in magnetohydrodynamic models of coronal condensations as more realistic radiation treatments are included. Additionally, we find that the line profiles for Hα differ dramatically during the cooling in a non-equilibrium ionisation model than for the same stratification using a statistical equilibrium treatment.