Time series of the noise-corrected disk average (over the disk region r/r_⊙ < 0.9) of the unsigned vertical flux density, smoothed with a 1-month wide time window (solid line). Due to an MDI data gap the solid line is absent in the interval 1998.4−1999.0. The dashed curve uses a second-order polynomial in the sunspot number (also smoothed with a 1-month time window) to fit the solid curve. This polynomial fit gives a basal flux density (in the absence of sunspots) of 2.7 G.