Probing the properties of active regions in the solar interface region using full-disc spectroheliograms

The composition of plasma in the solar corona is characterized by the first ionization potential (FIP) bias and is thought to be the result of a ponderomotive force acting in the chromosphere to separate ionized from neutral plasma. Identifying potential signatures of this fractionation process in the solar chromosphere continues to be the subject of active research. Full disc spectroheliograms of the chromosphere and transition region from the Interface Region Imaging Spectrometer (IRIS) spacecraft provide an opportunity to compare plasma signatures between active regions at different evolutionary stages and assess their relationship with the fractionation processes. Here, we compare different plasma properties from the C II, Si IV and Mg II lines observed by IRIS, finding no clear variability between active regions at different evolutionary stages in the C II and Si IV lines. However, distinct differences can be identified between the active regions at different evolutionary stages using the Mg II  k/h ratio (which provides a proxy for plasma opacity). In particular, the regions with the highest median FIP bias exhibit double peaked distributions of plasma opacity, suggesting variable plasma density, which could affect wave propagation in these locations. These results indicate that the relationship between the plasma properties and how the plasma is fractionated should be investigated in more detail by combining observations and modelling to better understand how it changes on both temporal and spatial scales. This article is part of the Theo Murphy meeting issue 'Solar atmospheric abundances in space and time'.