Variation with azimuthal angle in the plane of M 101 of the mean value of several variables, calculated in 30°-wide sectors in the radial ring . The azimuthal angle is counted counter clockwise from the northern major axis. Top panel: Equipartition magnetic field strengths B_tot (black dots), B_ran (red crosses) and B_ord (green circles). Upper middle panel: non-thermal polarization percentages p_n(6) and p_n(11). Lower middle panel: non-thermal depolarization DP_n(11, 6). Bottom panel: intrinsic rotation measure RM_i(11, 6). All error bars are statistical errors of one σ. The uncertainty in α_n causes systematic errors of 17% in B_ord, 20% in p_n(6), 12% in p_n(11), and 10% in DP_n(11, 6).