Upper panel: distribution of the magnetic field averaged in the 11 × 11 pixels boxes along the central meridian for three periods: May 1996–December 1997 (solid line, minimum), January 2000–December 2003 (dashed line, maximum), and January 2008–December 2010 (dotted line, minimum). Lower panel: same after correction from the noise level the correction method is described in Sect. 3.1.1 and is derived from Stenflo (2012).