Histograms of the speeds inferred from the four maps shown in Fig. 5, for the 2007/08/23 case (two top frames) and 2008/03/25 case (two bottom frames), with an average value of (63  ±  2) km s^-1 (corrected by projection effects) and (86  ±  3) km s^-1, respectively. The two corresponding histograms for each case refer to bin sizes of 5 km s^-1 (left) and 10 km s^-1 (right), respectively.