Line-of-sight Doppler shift and normal velocity of the oscillation along the loop. The blue curve shows the velocity of the oscillating loop in the normal direction at a snapshot of maximum velocity amplitude (at t = 200 s). The red line indicates the Doppler shifts from the synthesised spectral profiles, basically the vertical velocity in the loop, which also show signs of the field-aligned flows near the footpoints. The arc length has been normalised to the loop length, i.e. the footpoints in the photosphere are at 0 and 1, and the apex is around 0.5. See Sect. 3.4.