Chromospheric dynamics and the O I 135.6~nm spectral line

Abstract

The O I 135.6 nm spectral line is formed in the chromosphere at the same heights as the Mg II h&k line cores are formed. As the O I line is optically thin, it represents a possibility for measuring the non-thermal velocities in this region without the complications added by optically thick radiative transfer. Numerical models have hitherto strained to reproduce Mg II core line widths, challenging current understanding of chromospheric energetics and dynamics. We aim to construct numerical models, varying physical and numerical parameters in order to asses which of these is most important in setting the Mg II core intensity and width. A set of numerical models of varying resolution, size, magnetic topology and strength are considered and used to synthesize O I line emission and to investigate the constraints that observations of this line place on chromospheric dynamics and densities. We find that, for quiet Sun, while non-thermal motions undeniably provide a source of Doppler broadening and chromospheric mass loading, the average strength of the photospheric magnetic field is the most important parameter in setting the Mg II core width to values within 5 km/s of observed values. Furthermore, for plage, we identify non-equilibrium hydrogen ionization and three dimensional radiative transfer as important ingredients in understanding chromospheric diagnostics and deciphering chromospheric structure.