Long-term Variations of Solar Differential Rotation and Sunspot Activity: Revisited

Long-term variations of solar differential rotation and sunspot activity are investigated through re-analyzing the data on parameters of the differential rotation law obtained by Makarov, Tlatov, and Callebaut (1997), Javaraiah, Bertello, and Ulrich (2005a, b), and Javaraiah et al. (2009). Our results indicate that the solar surface rotation rate at the Equator (indicated by the A parameter of the standard solar rotation law) shows a secular decrease since cycle 12 onwards, given by about $1\,-\,1.5\times10^{-3}$($deg\ day^{-1} year^{-1}$). The B parameter of the standard differential rotation law seems to also show a secular decrease since cycle 12 onwards, but of weak statistical significance. The rotation rate averaged on latitudes ($0^{o}\,--\,40^{o}$) does not show a secular trend of statistical significance. Moreover, the average sunspot area shows a secular increase of statistical significance since cycle 12 onwards, while a negative correlation is found between the level of sunspot activity (indicated by the average sunspot area) and the solar equatorial rotation in the long run.