Sample of the steps followed in the analysis of the MDI magnetograms taken on Oct. 20, 2000, at 00:00 UT and at 01:36 UT for AR 9201. a) Average magnetogram corresponding to 00:48 UT on Oct. 20, 2000. Black and white correspond to negative and positive magnetic field, respectively. The saturation levels are  ± 1000 G; b) the arrows indicate the horizontal velocities measured by means of the DAVE method. The background image is the average magnetogram showed in a); c) helicity flux density map computed with the method of Pariat et al. (2005). Black and white correspond to negative and positive helicity flux density, respectively. The saturation levels are  ± 5  ×  10¹⁸ Mx² cm^-2 s^-1; d) elements of helicity flux determined by the YAFTA feature-tracking algorithm. The identified patches are numbered progresssively. The background image is the helicity flux density map showed in c) with saturation levels of  ± 2  ×  10¹⁸ Mx² cm^-2 s^-1. The field of view is of about 210  ×  140 arcsec².