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.

Diagnosing the Optically Thick/Thin Features Using the Intensity Ratio of Si IV Resonance Lines in Solar Flares

“…We find that the electron density is around 10 10.3 cm −3 in regions where R > 2, and is around 10 11.4 cm −3 in regions where R < 2. These results agree with that of Gontikakis & Vial (2018). Rose et al (2008) proposed that the specific geometry of the flaring plasma could also be a factor making the line ratio greater than 2.…”

“…The opacity effect cannot explain the cases where R is greater than 2. One possible explanation for this result is the decreased contribution of the collisional excitations when resonant scattering is dominant, as proposed by Gontikakis & Vial (2018), who reported a negative relation between line ratio and electron density. In this work, we have estimated the electron density from the ratio of O IV 1401.2 and 1399.8 Å lines.…”

“…This self-absorption feature is in-terpreted as a result of the overlying cool TR loops. Gontikakis & Vial (2018) also presented an observation that the intensity ratio between the Si IV 1394 and 1403 Å lines is greater than 2. This deviation is rendered to be the result of resonant scattering.…”

The Center-to-Limb Variation of Non-Thermal Velocities using IRIS Si IV

“…Assuming that these lines are optically thin, their ratio should be 2, which is equal to the ratio of the oscillator strengths for the two lines. Ratios lower than 2 indicate opacity effects, whilst ratios above 2 could be due to resonant scattering, as found by Gontikakis & Vial (2018) for 2 % of individual profiles in an active region.…”

Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

“…In the case of R2, there are many pixel locations where the ratio is larger than 2.1. It has been argued by Mathioudakis et al (1999) that opacity e⇧ects lead to ratios below 2, with a ratio of 1.8 corresponding to an optical depth ⇧ ⇤ 0.25, and by Gontikakis & Vial (2018) that values greater than 2 indicate a contribution from resonant scattering of Si ⇤⌅ radiation. However, detailed flare radiation hydrodynamics simulations by Kerr et al (2019) demonstrate that opacity e⇧ects can lead to a range of ratios from 1.8 to 2.3, corresponding closely to what we find, in cases where the flare energy carried by non-thermal electrons exceeds 5 ◊ 10 10 erg cm 2 s 1 and/or the electron spectrum is soft.…”