Detailed and simplified non-equilibrium helium ionization in the solar atmosphere

Abstract

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.

11 pages, 9 figures. Accepted for publication in ApJ

References in corpus (3)

• Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
• The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets
• Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results

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