ner_tags: O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O B-Wavelength O O O O O O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength O B-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O B-Telescope O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength B-Wavelength I-Wavelength I-Wavelength I-Wavelength O B-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O O B-Wavelength I-Wavelength O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model O O O O O O O O O O O B-Wavelength I-Wavelength I-Wavelength I-Wavelength O B-Wavelength I-Wavelength I-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: 4 . The two small circles in the NUV spectral window mark the downflow components in the Mg ii h and k lines. Figure 4. Average IRIS spectra at specific locations of two slits as marked in Figures 2 and 3 . The black lines represent the spectra of the typical plage region, whereas the green lines represent the spectra of a typical umbral region (marked on the right side of each panel of Figure 2 ). The red and blue lines are the average spectra of two regions where supersonic downflows are observed. These two regions are marked at the right side of each panel of Figures 2 and 3 , respectively. The red spectra show a downflow only in TR lines, whereas the blue spectra show a downflow in TR lines as well as in the chromospheric C ii and Mg ii h and k lines. Two arrows in the NUV spectral window mark the downflow components seen in the Mg ii h and k lines (spectra shown in blue). For the identification of different lines, we refer to Tian ( 2017 ). 3.2. Detection of Supersonic Downflows and Multi-Gaussian Fitting to Their Spectra To derive spectroscopic parameters of the supersonic downflows, we have selected a small spectral window of 1399.31–1403.89 Å, which contains the O iv 1400/1401 Å doublet and Si iv 1403 Å spectral lines. The primary reason for selecting this wavelength range is that Si iv 1403 Å is one of the strongest TR lines in the IRIS spectral range. Also, the O iv 1400/1401 Å doublet lines are useful to determine the electron density of the TR plasma. Figure 5 presents some examples of the spectra in the selected window. A total of six isolated components corresponding to the three spectral lines O iv 1400 Å, O iv 1401 Å, and Si iv 1403 Å and their satellite lines (due to supersonic downflows) can be seen at some locations. To increase the signal-to-noise ratio), we have made a three-point running smoothing along the wavelength and also along the slit dimensions. Figure 5. IRIS spectra containing two O iv lines and one Si iv line acquired along the vertical solid lines marked in Figure 1 . The spectral line profiles at the location indicated by the dashed line (also shown with the red dots in Figure 1 ) of each left panel are shown in the corresponding right panel. A six-component Gaussian fit to the spectra of the regular and downflow components of the O iv 1400/1401 Å and Si iv 1403 Å lines is shown as the overplotted blue line in each right panel. The fitting parameters (i—peak intensity, v—Doppler shift, w—line width) are also printed. The spectra within the red square box contain the Si iv line and its downflow component, which are used for the initial detection of the supersonic downflows (see the text for details). The detection of supersonic downflows is sometimes difficult, as the signal within sunspots is very weak (though it also depends on the exposure, binning of the spectra, etc.). Also, the TR is very dynamic and often shows broad line profiles due to different activities. To ensure that we primarily detect