Abstract :
The composition of the solar plasma is an important tool to study several physical processes, e.g. to trace back the source region of the solar wind. The most prominent effect is the change of elemental abundances from the photosphere to the corona according to the first ionisation potential, the FIP-effect. It is widely accepted that this takes place in the chromosphere, where the particles turn from neutral to ionised.
Many models for the element fractionation have been presented so far. Basically they all make use of the separation of ions and neutrals, but still, there is no conclusion as to which of them is the most important physical process leading to this separation. The situation is a bit like with coronal heating: many suggestions, but very limited ability to to prove them right or wrong.
Of course, the ideal way to test the models for the composition in the chromosphere and the transition region would be to perform direct observations of that region. But unlike in the corona a reliable inversion of the spectral observations of chromospheric and transition region emission lines is nearly impossible. But probably new thoughts on the structure of the chromosphere and the transition region combined with advanced forward modeling will finally lead to some new insights to this puzzling question.
