Effects of resonant scattering of the Si IV doublet near 140 nm in a solar active region

Abstract

Aims. In a previous study we analysed the C IV 1548.189 Å and 1550.775 Å lines observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER), showing cases where the 1548.189 Å spectral profile was noticeably different from the 1550.775 Å one, profiles that we dubbed differentially shaped profiles. We explained this differential behaviour by an important radiative contribution, affecting multiple plasma motions happening at the instrument sub-resolution scale. In the present study we examine more general cases where radiative effects may contribute to the emission from the transition region of an active region. Here we analyse the lines Si IV 1393.757 Å and 1402.772 Å observed with the Interface Region Imaging Spectrograph (IRIS). Methods. We study active region NOAA 12529, observed with IRIS on 18 April 2016. Using sorting techniques we selected individual profiles for which the intensity line ratio 1393.757 Å/1402.772 Å is significantly higher or lower than 2 and we also tracked differentially shaped profiles. We analyse the physical conditions that create these profiles and in some cases we estimate electron densities. Results. We found more than 4000 individual profiles with line ratios higher than 2, about 500 profiles for which the line ratio is in the range 1.3–1.6, and 15 differentially shaped profiles. Line ratios higher than 2, are found along loops, and mostly at the y = 250 to 300″ part of the plage. There, we estimated the incident radiation and derived electron densities that can vary from 109 to a few times 1011 cm−3, depending on the plasma temperature. For the low line ratios, the sources are concentrated at the periphery of the active region plage, mostly along fibrils and present optical depths, τ, between 1.5 and 3. in most cases. The electron densities calculated from these Si IV profiles are comparable with electron densities derived using the O IV 1399.766 Å-1401.163 Å ratios. Conclusions. We found that about 2.4% of the individual profiles for which we can perform a Gaussian fit present a line ratio higher than 2. In profiles with a high line ratio, the resonant scattering appears to be due to the combination of an average incident radiation field with a relatively low local electron density and not due to the vicinity of an ephemeral strong light source. As far as low intensity ratios are concerned, non-negligible optical depths are found at the edge of the plage, near the footpoints of fibrils that are oriented towards quiet Sun areas, where the electron density can be as high as (7 − 9) × 1011 cm−3 if we assume a plasma in ionization equilibrium.