tokens: in steps of 1′′ (a total field of view 15 × 119). The roll angle of the slits is –45° to the N–S direction on the Sun. Here, we focus on FUV spectra from both channels containing ∼1333–1337 Å (C ii lines), ∼1342–1344 Å (continuum), ∼1348–1351 Å (Fe xii line), ∼1353–1356 Å (Fe xxi and O i lines), and ∼1399–1406 Å (Si iv line), and on the NUV channel containing four spectral regions: 2791–2806 Å (Mg ii h and k lines), 2813–2816 Å, 2825–2828 Å, and 2831–2834 Å. IRIS also recorded SJIs for four passbands with a step of 2′′ and a total field of view 120 × 119: 1330 Å (C ii transition-region filter), 1400 Å (Si iv transition-region filter), 2796 Å (Mg ii k chromospheric filter), and 2832 Å (Mg ii wing photospheric imaging). 2.2. Solar Flare on 2014 March 29 An X1 flare observed on 2014 March 29 occurred in the active region NOAA 12017 with the GOES 1–8 Å flux starting to rise impulsively at 17:45 UT and peaking at 17:48 UT. IRIS started to observe the active region three hours before the flare, and the observations lasted a few minutes after the flare peak with the NUV exposure time of 8.00 s before 17:46:04.78 UT and 2.44 s after 17:46:13.98 UT, and a 75 s raster cadence (see e.g., Heinzel Kleint 2014 and Kleint et al. 2015 , 2016 ; Liu et al. 2015 ; Young et al. 2015 for detailed descriptions of IRIS observations of this flare). In contrast to the previous flare, the gradual phase was detected only at its beginning, and the flare loops are barely visible. Moreover, the slit rasters cover only the less pronounced parts of the loop system. We add this X-type flare because it was widely studied in the literature; however, our detailed analysis is focused mainly on IRIS spectra from the flare of 2015 June 22. To show the time lag between the flare ribbons and loops, light curves from a small part of the flare ribbons (red) and their corresponding loop tops (blue) in 1400 and 2796 Å are plotted in Figure 2 . It is seen that the loop tops peak about 3 minutes later than the ribbons in both 1400 and 2796 Å channels. Figure 2. Time lag between the flare ribbons and loops for 2014 March 29. Light curves from a small part of the flare ribbons (red lines) and their corresponding loop tops (blue lines) in 1400 and 2796 Å. Black lines represent the integral emission from the whole flare. Note that the sharp blue peak between 17:45 and 17:46 is due to leakage of saturated emission from the ribbon. 2.3. The Radiometric Calibration In our analysis, we used IRIS -calibrated data (Level 2 data), which are corrected for dark current, flat field, and geometry effects, and are wavelength calibrated. The radiometric calibration was performed in the following way. Intensities in the IRIS data are given in Data Number (DN) or counts (DN s −1 ). We perform radiometric calibration of the observed intensities to absolute units [erg s −1 cm −2 sr −1 Å −1 ]. To convert intensities to absolute units, we used the following calibration procedure:where I abs is the absolute
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