Time–distance diagrams of (a) the swirling strength λ, (b) the vertical component of the swirling vector λ_z, and (c) the vertical component of the magnetic swirling vector for the swirl event shown in Fig. 4. Values at each time step and height are averaged over a finite horizontal plane of 150 km side length. The paths of two test particles moving with Alfvén plus bulk speed (green) and with sound plus bulk speed (red) along the swirl are shown in panel b.