On the Long-Term Modulation of Solar Differential Rotation

Long-term modulation of solar differential rotation was studied with data from Mt. Wilson and our original observations during Solar Cycles 16 through 23. The results are: i) The global B-value (i.e. latitudinal gradient of differential rotation), is modulated in a period of about six or seven solar cycles. ii) The B-values of the northern and southern hemispheres are also modulated with a similar period to the global one, but iii) they show quasi-oscillatory behavior with a phase shift between them. We examined the yearly fluctuations of the B-values in every solar cycle with reference to the phase of the sunspot cycle and found that the B-values show high values over the full-cycle years, when the cycle-averaged B-values are high. We discuss the independent long-term behavior of solar differential rotation between the northern and southern solar hemispheres and its implication for the solar dynamo.