On the importance of Ca ii photoionization by the hydrogen lyman transitions in solar flare models

Author:

Osborne C M J1ORCID,Heinzel P2,Kašparová J2ORCID,Fletcher L13ORCID

Affiliation:

1. SUPA School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

2. Astronomical Institute of the CAS, CZ-25165, Ondrejov, Czech Republic

3. Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, NO-0135, Oslo

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.

Funder

Science and Technology Facilities Council

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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