Difference between even- and odd-numbered cycles in the predictability of solar activity and prediction of the amplitude of cycle 25
It was shown previously that the sunspot number (SSN) at a point 3 years before the minimum is well correlated with the maximum SSN of the succeeding cycle, and a better correlation is obtained when the maximum SSN is replaced by the average SSN over a cycle for which the average SSN is calculated by dividing cycles at a point 3 years before the minimum (Yoshida and Yamagishi, 2010; Yoshida and Sayre, 2012). Following these findings, we demonstrate in this paper that the correlation between the SSN 3 years before the minimum and the amplitude of the coming cycle differs significantly between even-numbered and odd-numbered cycles: the correlation is much better for even-numbered cycles. Further, it is shown that the amplitude of even-numbered cycles is strongly correlated with that of the succeeding odd-numbered cycles, while the correlation between amplitudes of odd-numbered cycles and those of succeeding even-numbered cycles is very poor. Using the excellent correlations, we estimate the maximum SSN of the current cycle 24 at 81.3 and predict the maximum SSN of cycle 25 to be 115.4 ± 11.9. It is of note, however, that a peak of the SSN has been observed in February 2012 and the peak value 66.9 is considerably smaller than the estimated maximum SSN of cycle 24. We conjecture that the second higher peak of the SSN may appear.