Rotational modulation of the longitudinal magnetic field of ζ Ori Aa for the observations taken in 2007–2008 (top) and 2011–2012 (center). The black line corresponds to the best dipolar fit, while the dashed blue line corresponds to the best quadrupolar fit. The bottom panel compares the fit of the dipole and the quadrupole obtained from the 2007–2008 data with the observations obtained in 2011–2012 (see Sect. 7.3). The data for which the Stokes V model matches the observed LSD V profiles are shown in black, while the data for which the Stokes V model does not match are in red.