Large-scale dynamics of active regions and small photospheric magnetic features

This paper presents an analysis of the dynamics of magnetic features within and outside active regions using full-disk magnetograms obtained by the MDI (Michelson Doppler Imager) instrument on the SOHO spacecraft. The absence of distortion and the short time interval (96 minutes) between images allow us to calculate high-quality cross-correlations between regions on each magnetogram. Therefore, each cross-correlation can be studied individually, which provides information with resolution in longitude as well as good temporal resolution. Rotation rates and meridional motions are simultaneously derived from an analysis of the cross-correlation functions. This analysis shows that the meridional motions are very different during the quiet period and the active period. In the former case, a poleward meridional circulation is observed, which increases with latitude. In the latter case, the meridional motions of magnetic features in active regions are smaller by a factor of 2 than those exhibited by small magnetic structures outside active regions. The rotation rate seems slightly larger in active regions, while the rotation rate decreases as the activity level rises. Some differences in the meridional circulation of different polarities are found.