Contours around active-region flows (thick black lines) during solar minimum (panel a) and during the rising phase (panel b), superimposed on SDO/HMI magnetograms (line-of-sight magnetic field). The thin black lines represent the contours of the magnetic mask discussed in Sect. 4.2. We only consider pixels whose great-circle distance from disk center is less than 50°, and hence the contours do not encompass the magnetic features near the limb.