Upper panel: mean longitudinal magnetic field of HD 50169 against rotation phase. The different symbols identify the source and/or instrumental configuration from which the ⟨B_z⟩ value was obtained, as follows: crosses (red): Babcock (1958); filled triangle (violet): Mathys & Hubrig (1997); filled circles (dark blue): Mathys (2017); filled squares (salmon): Romanyuk et al. (2014), and new measurements of spectra obtained with the Main Stellar Spectrograph of the 6 m telescope BTA of the Special Astrophysical Observatory. The long-dashed line (blue) is the best fit of the observations by a cosine wave and its first harmonic, see Eq. (5). 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_z⟩ 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.