Magnetic field maps (in grey scale) from SDO/HMI of a part of the active region 11158, on February 15, 2011, at 01:48:00 UT (top) and 02:00:00 UT (bottom), represented in the plane-of-sky. The orange and blue contours represent the positive and negative vertical electric current densities, respectively, with the magnitude greater than 100 mA/m². The black arrow in the top panel indicates the current ribbons extending from 195 arcsec to 230 arcsec in the x-direction, and lying between −220 arcsec and −230 arcsec in the y-direction. The red box shows the primary difference between the two maps: in this box, the total negative vertical current density varies from − 974.9 × 10⁹ A to − 1062 × 10⁹ A, and the total positive vertical current density varies from + 616.9 × 10⁹ A to + 762.4 × 10⁹ A. Therefore, the total increase of vertical current density is of 232 × 10⁹ A for the area of the red box. This represents an increase of 15% in the total current density in this area.