Cyclical Behaviors of Sunspot-Group Tilt Angles in Solar Cycles 21-23

Based on the Debrecen Photoheliographic Data (DPD) sunspot catalog, we investigate the cyclical behaviors of tilt angles of all sunspot groups (SGs) and SGs with angular separation constraint S > 2.5(?) in Solar Cycles (SCs) 21 - 23. It is found that, the cyclical behaviors of tilt angles during SC 23 are different from those of SCs 21 and 22, confirmed by using the SDD sunspot catalog where possible, which are embodied in the following five aspects: (i) For all SGs (SGs with separation constraint), the percentage of SGs with larger absolute values of tilt angles in the declining phase than during the maximum phase is about 80% (60%) in SCs 21 and 22. In SC 23, the percentage of SGs with larger absolute values of tilt angles during the declining phase is about 1.10 times (at least not lower than) that during the maximum. (ii) During SCs 21 - 23, the yearly mean of tilt angles decreases with time for all SGs (SGs with separation constraint) and the slopes of the linear regression lines are negative: -0.404, -0.910, and -0.740/-1.005 (-0.500, -1.138, and -0.764/-0.576). (iii) During SCs 21 and 22, the yearly mean of the absolute value of tilt angles generally decreases with time for all SGs (SGs with separation constraint) and the slopes of the linear regression lines also are negative: -0.347 and -0.451 (-0.504 and -0.397). (iv) During SC 23, for all SGs, the yearly mean of the absolute value of tilt angles generally increases with time and the slope of the linear regression line is positive: 0.281/0.429. For SGs with separation constraint, the linear regression line to the yearly mean of the absolute value of tilt angles is almost horizontal and its slope is -0.008/0.014. (v) The yearly mean of latitude of SGs decreases steadily with time during each SC.