Measurement of effective diffusion coefficients by the indirect method. Black: time evolution of the average vertical field strength for a box depth of 2.85 Mm. Initial magnetic field | B_z | = 50 G, polarity stripe width 3 Mm, horizontal resolution 25 km. Colors: Eq. (3) for five values of the assumed diffusion coefficient. Symbols mark the intersection points with the numerical evolution shown in Fig. 11.