Spectroscopic Measurements of Helium abundance in an Active Region

The chemical composition of the solar chromosphere is a key ingredient not only for models of the chromosphere itself, but also for understanding fractionation processes occurring elsewhere in the solar atmosphere (corona and solar wind), such as the so-called "FIP effect", where "FIP" stands for "First Ionization Potential". Being the most abundant element after hydrogen, and having the highest FIP, helium is a prime target for studying such issues. Here we report results of spectroscopic measurements and atmospheric modelling aimed at estimating the abundance of helium in the chromosphere of an active region, during a small C-class flare. We analyze the various sources of errors, and discuss the circumstances under which such an estimation becomes reliable, with an emphasis on the ability of estimating the helium-photoionizing EUV coronal flux, as well as the temperature and density structure of the underlying chromosphere.