Stacked plot of the O iv and Si iv line profiles as a function of time. The footpoint spectra cover the whole wavelength range and typically peak around 10 km s ^-1 redshift. The loop profiles are displayed only above 40 km s ^-1 (O iv) and 80 km s ^-1 (Si iv) to avoid confusion with the footpoint spectra. They peak well above 100 km s ^-1. The time the profiles are taken is color-coded (increasing from purple to red with an increment of 340 s and starting at 09:37 UT). The thin vertical line indicates zero Doppler shift. See Sect. 3.1.