Ratio between the photoionization rate and the collisional ionization rate from the hydrogen ground state, as a function of the electron density at height of 4 R_⊙ and effective thickness of 10 000 km. The temperatures are represented by colors and the flow velocities by symbols. At high temperatures, R_1k is fixed by the incident Lyman-continuum radiation, while the collisional rate is proportional to the electron density.