Demonstration of the procedure used to identify MCs. The magnetogram is a cut-out from the magnetogram of Fig. 3a. The same region is marked by the red lines as in Fig. 3. From panel a to panel b we show the processes of noise removal and the erode method in the clumping method. Panel c shows the identified MCs from panel b. All MCs have been colored (positive MCs: warm tones; negative MCs: cold tones).