Coalignment between the IRIS and SUMER observations. The dash-dotted line in the left panel in white represents the position of the SUMER slit. The right panel shows how the normalized integrated intensity changes along the slit. The blue line corresponds to the intensity of the pixels of the IRIS SJI along the white line from the bottom to the top. The black line corresponds to the Lyα integrated intensity at 09:13:33 UT along the SUMER slit.