Average line profiles of O iv and Si iv in selected regions. These regions are the loop footpoint (red), the loop itself (blue) and the background umbra (green) as indicated in Fig. 2e. The black dashed line shows an average quiet Sun spectrum adjacent to the sunspot (in the right panel scaled by 0.5 for better visibility). The top axes give the wavelength in Doppler shift units with respect to the rest wavelength of O iv and Si iv, respectively. The thin vertical line in both the panels corresponds to the respective zero Doppler shift. See Sect. 3.1.