HMI magnetic and deduced velocity fields of AR 11928 at different times. In all the panels, the gray levels show the vertical component, B_z, of the magnetic field and contours of B_z( ± 120 G) and the magnitude of the transverse vectors is indicated by the light blue segment located in the top left of each panel. Left panels: vector magnetic field observations indicate blue/red arrows above positive/negative B_z values. Vectors point to horizontal field direction and their length corresponds to the horizontal field magnitude (a zoom on a computer screen is needed to better view these arrows). Right panels: the horizontal velocity vectors (V_h) are shown with blue arrows. The velocity pattern in the leading polarity indicates the presence of a counter-clockwise rotating motion of the magnetic elements on 19 and 20 December 2013. The field of view is a portion of the actual observation indicated with white rectangular box in Fig. 2a.