The polar precursor method for solar cycle prediction: comparison of predictors and their temporal range
The polar precursor method is widely considered to be the most robust physically motivated method to predict the amplitude of an upcoming solar cycle.It uses indicators of the magnetic field concentrated near the poles around sunspot minimum. Here, we present an extensive performance analysis of various such predictors, based on both observational data (WSO magnetograms, MWO polar faculae counts and Pulkovo $A(t)$ index) and outputs (polar cap magnetic flux and global dipole moment) of various existing flux transport dynamo models.We calculate Pearson correlation coefficients ($r$) of the predictors with the next cycle amplitude as a function of time measured from several solar cycle landmarks: setting $r= 0.8$ as a lower limit for acceptable predictions, we find that observations and models alike indicate that the earliest time when the polar predictor can be safely used is 4 years after polar field reversal. This is typically 2--3 years before solar minimum and about 7~years before the predicted maximum, considerably extending the {usual} temporal scope of the polar precursor method. Re-evaluating the predictors another 3 years later, at the time of solar minimum, further increases the correlation level to $r\ga 0.9$. As an illustration of the result, we determine the predicted amplitude of Cycle 25 based on the value of the WSO polar field at the now official minimum date of December 2019 as $126\pm 3$. A forecast based on the value in early 2017, 4~years after polar reversal would have only differed from this final prediction by $3.1\pm 14.7$\%.