2026ApJ..1000..175C

We present the first spectropolarimetric time-series analysis of the Maunder Minimum analog HD 166620, using 12 nights of data from CFHT/SPIRou and a single epoch from CFHT/ESPaDOnS. While individual Stokes V profiles exhibit no significant polarization signatures, we leverage the rotational coverage of the SPIRou dataset to compute a grand average LSD profile. Forward modeling of the cumulative Stokes V signal, assuming a purely axisymmetric dipole, yields a best-fit dipole field strength of ${B}_{{\rm{dip}}}=1.1{0}_{-0.90}^{+0.95}$ G (3σ). This field strength matches simulations of the solar dipole during the Maunder Minimum phase. Our results are consistent with independent constraints on the dipole field strength from an LBT/PEPSI snapshot and exclude the presence of strong nonaxisymmetric fields potentially missed by this single-epoch observation. These findings provide direct empirical evidence that the transition to weakened magnetic braking involves a weakening of the large-scale magnetic field and suggest that HD 166620 represents a state comparable to the Sun near the peak activity of a grand minimum.