Time-slice contrast-enhanced intensity along a horizontal slit passing from the approximate visual centre of the swirl found within the red circle in Fig. 1. This figure corresponds to a cut-out of the respective time-slice panel (top second panel) of Fig. 9 in Paper I. The contrast-enhanced intensity corresponds to the Hα line-centre intensity convolved with the standard Interactive Data Language (IDL) emboss function, a procedure that helps to locate edges in an image. The overplotted red lines follow the temporal evolution of two different visually identified spiral arms of this swirl, and the dotted black lines indicate the linear trend of their radial motion.