LARGE SCALE FLOW AROUND A SUNSPOT
The flows start on either side of the sunspot and extend over 100 arcsec to the east. The general picture of the flows resembles the pattern of a hydrodynamic flow around an solid obstacle. The role of an obstacle is played by the large sunspot A. The MDI intensity movie (mpeg, 654K) shows that the small sunspots and pores, that have been formed along the moat boundary, flow around the sunspot in northeast or southeast directions, depending on their starting position. It can be seen in the right (western) part of Figure 2 , where arrows create a fan-like pattern.
The flow map suggests that these large-scale streams seem to occur due to the faster proper motion of the main sunspot relative to the ambient photospheric plasma. Because of the significant difference in mobility of different magnetic structures, the fast moving sunspot can resist the push of the oncoming photospheric plasma, while weaker objects are swept away by the flow. Our measurements show that the speed of small sunspots and pores, averaged over 44 hours, is about 100 m s-1. It has been known for a long time that sunspots rotate faster then the surrounding photosphere. The root-mean-square longitudinal drifts of sunspots are 0.67 - 0.76 degrees d-1, which corresponds to about 100 m s-1.