Structure of a photoionization layer in the solar chromosphere

We investigate the structure of a pure photoionization layer in the solar chromosphere on the basis of a one dimensional model in which an incident flux of EUV photons from above ionizes neutral hydrogen assumed to be flowing up with steady subsonic speeds from below. It is useful to analyze the structure equation in the phase plane of the hydrogen density (or velocity) and the ionizing photon flux. This shows that there is a unique critical solution which links the bottom of the layer to the top. Its structure is analogous to a classical constant pressure weak deflagration. The model is extended to include the ionization of minor species. In contradiction to previous work we note that one dimensional steady models are incapable of giving rise to a FIP (first ionization potential) effect by virtue of the fact that in a collision– dominated situation all minor species enter the layer at the hydrogen speed and exit at the proton speed and their fluxes are conserved.