The transition region in the Sun is the atmospheric layer that separates the cooler chromosphere from the hotter corona. Studying the structure and dynamics of the transition region requires greater attention as these provide critical information on the supply of mass and energy from the lower atmosphere to the corona and the solar wind. Emission lines originating in the transition region mostly fall in the ultraviolet part of the spectra. These UV photons can propagate through the upper atmosphere without significant absorption, re-emission and scattering. Thus transition region emission lines are generally considered as optically thin. However, some of the transition region lines are affected by opacity effects.
How do we determine if the emission line is optically thin or thick?
Under optically thin condition, the ratio of intensities of two lines originating from the same upper level is simply the ratio of their transition probabilities. The strongest lines as recorded by the Interface Region Imaging Spectrograph are two Si IV lines, though we note that the two Si IV lines originate from two different levels, in optically thin conditions, the intensity of the Si IV 1394 line is twice the intensity of the Si IV 1403 line.
The main aim of this paper is to study the opacity effects on the two Si IV lines in an emerging flux region (EFR) and how these effects vary during the evolution of the active region. Figure 1 displays three snapshots of the emerging active region as was observed with the Solar Dynamic Observatory simultaneously in the 193A passband (due to Fe XII) and photospheric magnetic field.
Throughout the development of the EFR, the histograms of the ratio are asymmetric (skewed as well as shifted) towards the low ratio values (see Figure 2). This tendency is stronger during the initial stage of flux emergence. Both the average and median of the distribution were lower than 2 in the initial phase, and they gradually approached to 2, which is the theoretically expected value for optically thin plasmas. On the other hand, in the quiet Sun, the distribution is highly symmetric and peaks at 2. There are also significant number of pixels with ratios larger than 2. Our analysis shows that the pixels with ratios smaller than 2 are predominantly located at the periphery of the active region whereas those with ratios larger than 2 are in the core. The reduced Si IV ratio is usually attributed to optical effect, as the line with the largest oscillator strength is strongly affected by opacity. In EFRs, particularly during the early phase, dense chromospheric plasma is lifted to the corona by emerging loops. Heating of the plasma often occurs intermittently due to the filamentary structure in the rising magnetic flux. The magnetic reconnection between the neighbouring rising loops also causes small scale brightenings in and above the chromospheric heights. Such dynamic events increase the densities along the line of sight resulting in reabsorption of photons by Si IV ions. Since the probability of reabsorption for the 1394A line is a factor 2 greater than for the 1403A line, so its intensity gets reduced more and the ratios less than 2 is observed. The results for R>2 suggests that the brightness of a low-density region next to a bright could be significantly enhanced by scattered photons.
We conclude that the Si IV lines observed in active regions are affected by the opacity during the early phase of the flux emergence. The results obtained here could have important implications for the modelling of the solar atmosphere, including the initial stage of the emergence of an active region as well as quiet Sun. During the early phase of the development, the majority of the pixels show intensity ratios smaller than two. However, as the active region evolves, more and more pixels show the ratios closer to two. Besides, there are a substantial number of pixels with ratio values larger than 2. At the evolved stage of the active region, the pixels with ratios smaller than two were located on the periphery, whereas those with values larger than 2 were in the core.
Full details see: Durgesh Tripathi, Nived Vilangot Nhalil, Hiroak Isobe & Gerry Doyle (link to article on arxiv.org)