Comparison between cycle-to-cycle variations in the coefficient of Joy’s law and covariance of rotation residuals and meridional motions of sunspot groups

ABSTRACT The tilts of bipolar magnetic regions are believed to be caused by the action of Coriolis force on rising magnetic flux tubes. Here, we analysed the combined Greenwich and Debrecen observatories sunspot-group data during the period 1874–2017 and the tilt angles of sunspot groups measured at Mt. Wilson Observatory during the period 1917–1986 and Debrecen Observatory during the period 1994–2013. We find that there exists about eight-solar cycle (Gleissberg cycle) trend in the long-term variation of the slope of Joy’s law (increase of tilt angle with latitude). There exists a reasonably significant correlation between the slope/coefficient of Joy’s law and the slope (namely, residual covariance) of the linear relationship between the rotation residuals and meridional motions of sunspot groups in the Northern hemisphere and also in the Southern hemisphere during Solar Cycles 16–21. We also find that there exists a good correlation between north–south difference (asymmetry) in the coefficient of Joy’s law and that in the residual covariance. We consider the residual covariance represents tentatively the coefficient of angular momentum transport. These results suggest that there exists a relationship between the surface/subsurface poleward/equatorward angular momentum transport and the Joy’s law. There is a suggestion of the strength of the Joy’s law depends on the strength of the poleward angular momentum transport.