Abstract
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 modelled to obtain the correct interpretation of observations. Here, we investigate the importance of photoionization 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 photoionization 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 per cent change in detailed optically thick radiative losses from considering these photoionization effects on the calcium lines in the two simulations presented, demonstrating the importance of considering these effects in a self-consistent way.
Highlights
Radiation-hydrodynamic (RHD) simulations are a common approach to modelling solar flares against which observations can be compared, and theoretical predictions investigated
We have shown that the inclusion of the radiation field from the hydrogen Lyman lines in the calculation of the calcium populations substantially change both the emergent calcium line profiles, and the formation regions of these lines
The FLARIX results were inserted into a Lightweaver model and similar relative differences between Lyman inclusive (LI) and Lyman exclusive (LE) for the Ca II 854.2 nm line were found
Summary
Radiation-hydrodynamic (RHD) simulations are a common approach to modelling solar flares against which observations can be compared, and theoretical predictions investigated. The hydrogen Lyman lines are very strongly enhanced in observations of flares, where a two order of magnitude enhancement between quiet sun and flaring region was found by Rubio Da Costa et al (2009) using the Transition Region and Coronal Explorer (TRACE), and in RHD simulations using RADYN which suggest similar or even larger enhancements (Brown et al 2018; Hong et al 2019) These highly enhanced lines lie in a wavelength range spanned by several Ca II to Ca III continua and provide a mechanism for Ca II photoionization, possibly influencing the opacity throughout the chromosphere and in turn the energy balance and emergent calcium spectral-line profiles. We will investigate whether these effects change the Ca II radiative losses sufficiently to modify the energy balance of the model
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