Non-equilibrium ionization

A frequent assumption in modeling the ISM/CGM is that the plasma is in collisional or photo-ionization equilibrium. This means that the ionization state of a plasma is in equilibrium (net ionization rate equals the net recombination rate) and can be represented by the temperature of the plasma and the local radiation field. Such assumptions are generally valid when the ionization and recombination time scale is much shorter than the thermal time scale (the time scale over which the temperature changes). The assumption breaks down when the plasma temperature changes much faster than it can ionize or recombine. In such cases, the ionization state lags behind the instantaneous temperature. The ionization state of the plasma is then known as the non-equilibrium ionization state. There is no prior way of calculating the non-equilibrium effects since the ionization state in such cases is dependent on the history of the plasma. 

In the last few years, I have developed/modified a code that can track the ionization state of plasma along with solving the hydrodynamical fluid equations.  I have shown in a few of my works that such assumptions of ionization equilibrium may result in misleading interpretations of the observing system.