On the Importance of Ca II Photoionisation by the Hydrogen Lyman Transitions in Solar Flare Models

TL;DR

This study shows that photoionisation of Ca II by hydrogen Lyman lines significantly affects spectral line profiles and energy balance in solar flare models, emphasizing the need for self-consistent radiative transfer calculations.

Contribution

It demonstrates the importance of including Ca II photoionisation by hydrogen Lyman transitions in solar flare simulations for accurate spectral and energy modeling.

Findings

Ca II 854.2 nm line profiles are significantly affected by photoionisation.

Considering these effects results in 10-15% change in radiative losses.

Photoionisation impacts the interpretation of flare observations.

Abstract

The forward fitting of solar flare observations with radiation-hydrodynamic simulations is a common technique for learning about energy deposition and atmospheric evolution during these explosive events. A frequent spectral line choice for this process is Ca II 854.2 nm due to its formation in the chromosphere and substantial variability. It is important to ensure that this line is accurately modeled to obtain the correct interpretation of observations. Here we investigate the importance of photoionisation of Ca II to Ca III by the hydrogen Lyman transitions; whilst the Lyman continuum is typically considered in this context in simulations, the associated bound-bound transitions are not. This investigation uses two RADYN flare simulations and reprocesses the radiative transfer using the Lightweaver framework which accounts for the overlapping of all active transitions. The Ca II 854.2 nm line profiles are found to vary significantly due to photoionisation by the Lyman lines, showing notably different shapes and even reversed asymmetries. Finally, we investigate to what extent these effects modify the energy balance of the simulation and the implications on future radiation-hydrodynamic simulations. There is found to be a 10-15% change in detailed optically thick radiative losses from considering these photoionisation effects on the calcium lines in the two simulations presented, demonstrating the importance of considering these effects in a self-consistent way.