Solar dynamics over solar cycle 21 using sunspots as tracers. I. Sunspot rotation.

Sunspots observed on spectroheliograms have been used as tracers to derive the rotation of the convective layers. Image-processing and pattern recognition technique were developed to infer the position of the tracer, thereby its motions, with a good accuracy. Some new properties of sunspot rotation have been detected, concerning the amplitude of the differential rotation and its variability through the solar cycle. Comparison is made with similar studies which do not offer the same accuracy. The results presented here give a hint of the depth where the magnetic field of sunspots is anchored. This is of importance for a better understanding of the origin of the differential rotation and of the solar dynamo