Si iv spectral profiles observed along the vertical line in Fig. 4. Each spectrum is normalised to its maximum and shifted in y-direction by a constant amount to create this stack plot. The y-axis indicates the spatial extent along the vertical line in Fig. 4. The coloured vertical bars associated with each spectrum show the position of the intensity-weighted Doppler shift of the line, while the colour of the bar indicates the line intensity, according to the colour bar above the plot. Vertical lines indicate zero and ±50 km s^-1 Doppler shifts. See Sect. 2.3.