Variations in solar rotation with the sunspot cycle

The positions of sunspots as photographed in white light at Mt. Wilson from 1921-82 were analyzed to detect any systematic variations, particularly in relation to the solar cycle. The study analyzed 5 deg sq bins of sunspots for both hemispheres. The residual rotation rates were calculated for individual and grouped sunspots. Peaks in the sunspot rotation rate were detected near the solar maximum and minimum and in high solar latitudes 3 yr before the end of the cycle. The highest sunspot rotation peaks were 0.5 deg/day. The ubiquity of the rotation rate changes over the whole solar disk implied periodic angular momentum exchanges between the photosphere and deeper layers of the convective zone. Finally, the interpretations of the differences observed between variations in sunspot and Doppler rotation are discussed.