Examples of the observations analysed in this study taken on 01 April 2011 at 09:06:00 UT. Panel a: SDO/HMI unsigned B_LOS magnetogram. Panel b: SDO/HMI continuum contrast image (i.e. compensated for intensity CLV). Panel c: Rome/PSPT Ca II K. Panel d: red continuum contrast images. The grey-scale bars on the right-hand side of each panel show magnetic signal in G and contrast, respectively. The squares indicate the insets shown in Fig. A.1.