Upper panel: mean quadratic magnetic field of HD 166473 against rotation phase. The symbols have the same meaning as in Fig. 3. The long-dashed line (red) is the best fit of the CASPEC measurements of Mathys (2017) and of the HARPS and ESPaDOnS data of this paper by a cosine wave (see Eq. (6)). The short-dashed line (green) corresponds to the superposition of low-order multipoles discussed in Sect. 4. Lower panel: differences O–C between the individual ⟨B_q⟩ measurements and the best-fit curve, against rotation phase. The dotted lines (blue) correspond to ±1 rms deviation of the observational data about the fit (red dashed line). The symbols are the same as in the upper panel. The CASPEC measurements of Mathys & Hubrig (1997) are not shown in this panel as they were not included in the fit (see text).