Evolution of total unsigned magnetic flux and flux imbalance in NOAA AR 11726 derived from the processed and re-projected SDO/HMI B_z magnetogram series (Sect. 3.2), where the pixels |B|< 250 Mx cm⁻² were masked to zero to avoid spurious effects arising from the noise-dominated weak-field pixels. The black curve shows the evolution of the unsigned flux over the entire active region patch, whereas the green curve shows the unsigned flux evolution of the small western bipole (see Fig. 1) multiplied by a factor of 30 to make its scale comparable to the unsigned flux of the entire active region. The red dashed curve shows the relative flux imbalance of the active region. The vertical dotted line shows the time of the weak CME eruption of 22 April 2013, 08:40 UT that we focus on in this study.