| Abstract : | To predict the strength of an upcoming solar cycle, we usually apply the polar precursor method, which is probably the most robust and physics-based method. It uses the magnetic field concentrated near the poles around the solar cycle minimum. We have done an extensive performance analysis of various such predictors, based on both observational and dynamo model data. We calculate linear correlation coefficients (r) of the predictors with the next cycle amplitude as a function of time measured from solar cycle maximum and polar field reversal. 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 to 3 years before the solar minimum and about 7 years before the predicted maximum i.e. considerably extending the usual temporal range of the polar precursor method. Reevaluating the predictors after 3 years, at the time of solar minimum, further increases the correlation level to r > 0.9. The predicted amplitude of Cycle 25, based on the value of the WSO polar field at the official minimum date (December 2019) is 126 ± 3. A forecast based on the value in early 2017 i.e. just 4 years after the polar reversal would have only differed from this final prediction by about 3%. |
