Temporal evolution of the Doppler velocity, inferred from the GFPI Fe I 5435 Å restored images, during the appearance of a two-armed spiral wavefront. Blue (red) color indicates upflows (downflows). Velocities are saturated for amplitudes higher than 300 m s⁻¹. The black lines mark the umbral and penumbral boundaries, as determined from contours of constant intensity. The green circles highlight the location of the spiral wavefronts. The time in the top-left corner of each panel shows the seconds since the beginning of the observations (08:10:08 UT). A movie is provided in online.