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We analyzed the sunspot-group daily data that were reported by GreenwichPhotoheliogrphic Results (GPR) during the period 1874-1976 and DebrecenPhotoheliographic Data (DPD) during the period 1977-2017. We determined theequatorial rotation rate [A] and the latitude gradient [B] components of thesolar differential rotation by fitting the data in each of the 3-year movingtime intervals (3-year MTIs) successively shifted by one year during the period1874-2017 to the standard law of differential rotation. The values of A and Baround the years of maxima and minima of Solar Cycles 12-24 are obtained fromthe 3-year MTIs series of A and B and studied the long-term cycle-to-cyclemodulations in these coefficients. Here we have used the epochs of the maximaand minima of Solar Cycles 12-24 that were recently determined from the revisedVersion-2 international sunspot-number series. We find that there exits aconsiderably significant secular decreasing-trend in A around the maxima ofsolar cycles. There exist no secular trends in both A and B around the minimaof solar cycles. The secular trend in B around the maxima of solar cycles isalso found to be statistically insignificant. We fitted a cosine function tothe values of A, and also to those of B, after removing the correspondinglinear trends. The cosine-fits suggest that there exist ~54-year (~94-year) and~82-year (~79-year) periodicities in A (B) around the maxima and minima ofsolar cycles, respectively. The amplitude of the cosine-profile of A (B) aroundthe minima of solar cycles is about 41% (65%) larger than that of A (B) aroundthe maxima. In addition, the cosine profiles of A and B suggest a large (up to180 degree) phase difference between the long-term variations of A, and alsobetween those of B, around maxima and minima of solar cycles.

solar physics

Long-Term Variations in Solar Differential Rotation and Sunspot  Activity, II: Differential Rotation Around the Maxima and Minima of Solar  Cycles 12-24