Histograms of the Doppler shifts of the Si IV line. The frequency (a) and cumulative frequency (b) of the Doppler shifts of the Si IV line after integration over 30 s as shown in Fig. 3a–c. The solid black, red, and blue lines indicate the distribution of the Si IV line at the original, IRIS, and SUMER resolution, respectively. The red dashed line indicates the distribution of the Si IV line calculated from the observation shown in Fig. 1. The vertical dotted line shows the zero shift. See Sect. 3.3 for more details.