Partially-ionised two-fluid shocks with collisional and radiative ionisation and recombination - multi-level hydrogen model

Abstract

Explosive phenomena are known to trigger a wealth of shocks in warm plasma environments, including the solar chromosphere and molecular clouds where the medium consists of both ionised and neutral species. Partial ionisation is critical in determining the behaviour of shocks, since the ions and neutrals locally decouple, allowing for substructure to exist within the shock. Accurately modelling partially ionised shocks requires careful treatment of the ionised and neutral species, and their interactions. Here we study a partially-ionised switch-off slow-mode shock using a multi-level hydrogen model with both collisional and radiative ionisation and recombination rates that are implemented into the two-fluid (PIP) code, and study physical parameters that are typical of the solar chromosphere. The multi-level hydrogen model differs significantly from MHD solutions due to the macroscopic thermal energy loss during collisional ionisation. In particular, the plasma temperature both post-shock and within the finite-width is significantly cooler that the post-shock MHD temperature. Furthermore, in the mid to lower chromosphere, shocks feature far greater compression then their single-fluid MHD analogues. The decreased temperature and increased compression reveal the importance of non-equilibrium ionised in the thermal evolution of shocks in partially ionised media. Since partially ionised shocks are not accurately described by the Rankine-Hugoniot shock jump conditions, it may be incorrect to use these to infer properties of lower atmospheric shocks.