The solar chromosphere is the dynamic interface between the photosphere
and the corona. In this region the gas transitions from a near neutral to
a highly ionized state. The ionization state of the gas is important for the energy
balance, affecting important quantities such as the gas temperature, pressure and
electron density. Hydrogen and helium, being the most abundant elements in the
solar gas, will have the biggest impact on the energy balance. The ionization-recombination
timescales of these elements is in the chromosphere long or comparable to the
dynamical timescale. We want to build a numerical model of the solar atmosphere,
and we want to include the effects of the long ionization-recombination timescales.
A detailed description of the relevant ionization processes is too computationally costly
to to include in a multidimensional model, so we have developed simplified descriptions
and implemented them in the code Bifrost. I will go through the these simplified descriptions
of hydrogen and helium and show how the non-equilibrium ionization changes the structure
of the atmosphere.