PHOTOSPHERIC PLASMA FLOWS AROUND A SOLAR SPOT VASYL B. YURCHYSHYN ∗ and HAIMIN WANG Big Bear Solar Observatory, Big Bear City, CA 92314, U.S.A. (e-mail: vayur@bbso.njit.edu) (Received 2 January 2001; accepted 2 August 2001) Abstract. We study photospheric plasma flows in an active region NOAA 8375, by using unin- terrupted high-resolution SOHO/MDI observations (137 intensity images, 44 hours of observations). The active region consists of a stable large spot and many small spots and pores. Analyzing horizontal flow maps, obtained with local correlation tracking technique, we found a system of stable persistent plasma flows existing in the active region. The flows start on either side of the sunspot and extend over 100 ′′ to the east. Our measurements show that the speed of small sunspots and pores, averaged over 44 hours, was about 100 m s -1 , which corresponds to root-mean-square longitudinal drifts of sunspots of 0.67 ◦ –0.76 ◦ day -1 . We conclude that these large-scale flows are due to faster proper motion of the large sunspot relative to the ambient photospheric plasma. We suggest that the flows may be a good carrier to transport magnetic flux from eroding sunspots into the outer part of an active region. 1. Introduction Distribution of observed magnetic and intensity structures on the surface of the Sun and in its chromosphere and corona depends on the nature of both large scale (super and mesogranules) and small-scale (granules) subsurface flows (Leighton, 1964; Simon and Leighton, 1964). Coupling of the photospheric plasma flows and the magnetic field distribution, as well as the fact that the footpoints of chromospheric and coronal loops are located in the photosphere, means that the plasma flows lead to continuous restructuring of the photospheric magnetic fields and strongly affect the formation and evolution of the coronal field. Interaction of the magnetic fields with the granulation and supergranulation flows causes the photospheric magnetic flux of an active region to be dispersed over the solar surface on a time scale of days to months. It is also clear that the magnetic fields, in turn, have a great effect on the flow pattern. Inspections of flow maps showed that the size of network cells in plages is significantly smaller than that in the quiet Sun (Zwaan, 1978). The variety of observed flows around sunspots and pores (Simon et al., 1988; Wang and Zirin, 1992) suggests that magnetic structures affect the flow of the photospheric plasma. Schrijver et al. (1996) showed that mobility of the magnetic elements (pores, small and large sunspots) depends on the net flux in them. Decrease of mobility with increasing flux then would imply that stronger magnetic fluxes should modify surrounding flow pattern. However, little is known on how a strong sunspot would ∗ Also at Crimean Astrophysical Observatory, 98409, Nauchny, Crimea, Ukraine. Solar Physics 203: 233–238, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.