  Helium ionization plays an important role in the energy balance of the upper
chromosphere and transition region. Helium spectral lines are also often used
as diagnostics of these regions. We carry out 1D radiation-hydrodynamics
simulations of the solar atmosphere and find that the helium ionization is
mostly set by photoionization and direct collisional ionization, counteracted
by radiative recombination cascades. By introducing an additional recombination
rate mimicking the recombination cascades, we construct a simplified 3 level
helium model atom consisting of only the ground states. This model atom is
suitable for modeling non-equilibrium helium ionization in 3D numerical models.
We perform a brief investigation of the formation of the He I 10830 and He II
304 spectral lines. Both lines show non-equilibrium features that are not
recovered with statistical equilibrium models, and caution should therefore be
exercised when such models are used as a basis in the interpretation of
observations.
