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

Studies of variations in the solar differential rotation are important for understanding the underlying mechanism of solar cycle and other variations of solar activity. We analyzed the sunspot-group daily data that were reported by Greenwich Photoheliographic Results (GPR) during the period 1874 - 1976 and Debrecen Photoheliographic Data (DPD) during the period 1977 - 2017. We determined the equatorial rotation rate [A ] and the latitude gradient [B ] components of the solar differential rotation by fitting the data in each of the 3-year moving time intervals (3-year MTIs) successively shifted by one year during the period 1874 - 2017 to the standard law of differential rotation. The values of A and B around the years of maxima and minima of Solar Cycles 12 - 24 are obtained from the 3-year MTIs series of A and B and studied the long-term cycle-to-cycle modulations in these coefficients. Here we have used the epochs of the maxima and minima of the Solar Cycles 12 - 24 that were recently determined from the revised Version-2 international sunspot-number series. We find that there exits a considerably significant secular decreasing-trend in A around the maxima of solar cycles. There exist no secular trends in both A and B around the minima of solar cycles. The secular trend in B around the maxima of solar cycles is also found to be statistically insignificant. We fitted a cosine function to the values of A , and also to those of B , after removing the corresponding linear 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 of solar cycles, respectively. The amplitude of the cosine-profile of A (B ) around the minima of solar cycles is about 41% (65%) larger than that of A (B ) around the maxima. In addition, the cosine profiles of A and B suggest a large (up to 180^∘) phase difference between the long-term variations of A , and also between those of B , around maxima and minima of solar cycles. Implications of all these results are discussed briefly.