Top: vector magnetic field map, bottom: vertical electric current density map of the active region NOAA AR 11158; produced from HMI data, on February 15, 2011, at 01:48:00 UT, and represented in the plane-of-sky (see text and Appendix A for more details). The rectangular box represents the field of view used in the present study. Top: the arrows represent the horizontal component of the field (for horizontal component greater than 100 G), and the colours represent the intensity of the vertical magnetic field (see scale). Bottom: vertical component of the electric current density (see colour scale).