tokens: (denoted by the cyan diamond) where the spectra of Si iv , C ii , and Mg ii are fairly bright and show some blueshifts. Figure 1. Observation overview of the B1.6 flare event on 2016 December 6. Top left panel: an AIA 131 Å image during the rise phase of the flare. The green dotted line and the purple contours mark the IRIS slit position and the flaring footpoints, respectively, which are the same as in the SJI 1400 Å image (bottom left panel). Top right panel: light curves of GOES 1–8 Å (black), AIA 131 Å (blue, arbitrary scale), and SJI 1400 Å (red, arbitrary scale) from the flaring region. The vertical black line represents the peak time of the flare. The blue and red dashed lines indicate the times when the AIA 131 Å and SJI 1400 Å images were observed, respectively. The two red dotted lines mark the time range (10:37:00–10:39:00 UT) as shown in Figures 3 and 4 . Bottom right panel: spectra of Si iv , C ii , and Mg ii along the slit at 10:37:45 UT. The vertical dotted line represents the reference wavelength for each of the spectral lines. The cyan diamond denotes the footpoint location that is cut across by the slit, as also shown in the SJI 1400 Å image. The two horizontal lines mark the space range (10′′) that is plotted in Figure 3 . 3.1.2. Blueshifts in the Si <sc>iv</sc>, C <sc>ii</sc>, and Mg <sc>ii</sc> Lines Figure 2 shows the Si iv , C ii , and Mg ii line profiles at the footpoint location denoted by the diamond (see Figure 1 ) for two times. It is seen that all the three lines are blueshifted, especially at an earlier time (10:37:34 UT, top panels). The blueshift velocities are measured to be 20, 13, and 6.3 km s −1 for the Si iv , C ii , and Mg ii lines, respectively, namely decreasing as the line formation temperature decreases. A few seconds later (10:37:45 UT, bottom panels), all the velocities decrease nearly by half. Note that some of the Mg ii (and also C ii ) line profiles are almost singly peaked, i.e., without a central reversal, which is a common feature for flaring footpoints. Figure 2. Si iv , C ii , and Mg ii line profiles at the footpoint location denoted by the cyan diamond as shown in Figure 1 for two times. The dotted line in each panel represents the reference wavelength of the spectral line. For the Si iv line, we adopt a single Gaussian fitting (the red curve) to derive the Doppler velocity ( V g ). For the optically thick C ii and Mg ii lines, we make a moment analysis to obtain the Doppler velocity ( V m ). The spatio-temporal variations of the total intensity, Doppler velocity, and line width of the Si iv and Mg ii lines are plotted in Figure 3 . From the intensity map we can see that the bright footpoints (the contours) show some apparent motion toward the north. Interestingly, at the footpoints (e.g., the ones denoted by the diamond), both of the Si iv and Mg ii lines exhibit blueshifts particularly at an early time. Such a blueshift in cool lines has scarcely been reported before. A similar
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