Dynamic Hydrogen Ionization

TL;DR: In this article, the authors investigate the ionization of hydrogen in a dynamic solar atmosphere and show that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state.

Abstract: We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons—the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there.