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

Abstract

The composition of plasma in the solar corona is characterised by the First Ionisation Potential (FIP) bias, and is thought to be the result of a ponderomotive force acting in the chromosphere to separate ionised from neutral plasma. Identifying potential signatures of this process in the solar chromosphere is the subject of active research. Full disk 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 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 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

Figures (7)

• Figure 1: Full disk mosaics taken by the IRIS spacecraft in the Si IV 1394 Å (top row) and Si IV 1403 Å (bottom row) spectral lines. Each spectral line was fitted using a single Gaussian, with the left column showing the fitted peak intensity, the centre column the Doppler velocity, and the right column the line width. Panel a shows the active regions defined by Mihailescu:2022 and analysed here, with panel d showing the leading (red) and following (black) polarities of the corresponding active regions.
• Figure 2: Full disk mosaics taken by the IRIS spacecraft in the C II 1334 Å and C II 1335 Å spectral lines, with the lines indicated by the text on the right hand side of each row. The upper two rows show the lines as fitted using a single Gaussian, while the bottom two rows show the results derived using a quartile approach (see text for details). In each case, the left column shows the fitted peak intensity, the centre column the Doppler velocity, and the right column the full width at half maximum (FWHM) of the line (to enable a direct comparison between the Gaussian fitting and quartile techniques). Panel a shows the active regions defined by Mihailescu:2022 and analysed here, with panels d, g, and j showing the leading (red) and following (black) polarities of the corresponding active regions.
• Figure 3: Full disk mosaics in the Mg II h & k lines showing the derived h/k 3 velocity (top row) and h/k 2 separation (bottom row) for the Mg II h (left column) and k (right column) lines. The regions of interest have been indicated on panel (a) as in Figures \ref{['fig:siiv_context']}a and \ref{['fig:cii_context']}a.
• Figure 4: Full disk mosaics showing the Doppler velocity (panels a & d), line width (panels b & e), and line asymmetry (panels c & f) for the Mg II k (top row; panels a, b, c) and Mg II h (bottom row; panels d, e, f) lines. Panel g shows the ratio of integrated intensity across the k and h lines. Panel a also shows the different active regions analysed here for context.
• Figure 5: Kernel density estimation (KDE) plots showing the variability in Doppler velocity (upper three rows) and FWHM (bottom three rows) for the C II 1334 Å (assuming both a Gaussian fit and quartile approach) and Si IV 1394 Å lines. In each case, the wavelength is indicated by the labels on the right hand side, with the regions of interest shown by the label on the bottom row. The Doppler velocity distributions are centred on zero, with the FWHM mostly distributed about 0.1 - 0.2 Å regardless of technique used to derive it. Note that the C II 1335 Å and Si IV 1403 Å lines display comparable behaviour to the C II 1334 Å and Si IV 1394 Å lines respectively and are not shown here for clarity.