On the Relation between Photospheric Flow Fields and the Magnetic Field Distribution on the Solar Surface


Using the technique of local correlation tracking on a 28 minute time sequence of white-light images of solar granulation, we have measured the horizontal flow field on the solar surface. The time series was obtained by the Solar Optical Universal Polarimeter (SOUP) on Spacelab 2 (Space Shuttle flight 5I-F) and is free from atmospheric blurring and distortion. The SOUP flow fields have been compared with carefully aligned magnetograms taken over a nine hour period at the Big Bear Solar Observatory (BBSO) before,, during. and after the SOUP images. The flow field and the magnetic field agree in considerable detail: vectors dc which define the flow of the white-light intensity pattern (granulation) point toward magnetic field regions, magnetic fields surround flow cells, and magnetic features move along the flow arrows, The projected locations of free particles ("corks") in the measured flow field congregate at the same locations where the magnetic field is observed. Subject headings: Sun: atmospheric motions Sun: granulation Sun: magnetic fields L INUTODUCTION (supergranular, mesogranular) and small-scale (granular) sub. Simon and Leighton (1964) suggested that the solar supersurface flows. granulation flow field (Leighton, Noyes, and Simon 1962) On the observational side, Simon (1%7) showed evidence in carries the solar magnetic field to the flow cell boundaries to a statistical study of the motions of some 2300 granules durin form the well-known chromospheric network pattern long a 25 minute time series that individual granules tend to flow seen in calcium and hydrogen images. Leighton (1964) also toward supergranule boundaries. Using much better data, created a diffusion model of the solar cycle in which the interMuller and Mena (1987) have recently succeeded in direc*y action of the flow field and the magnetic field through a observing outflows of granules and facular points from samrandom walk process was responsible for the distribution of spots on Pic du Midi images. There have also been many the magnetic field along the solar surface over the solar cycle, attempts to measure the motions of individual magnetic fidd Later theoretical studies of magnetoconvection (Weiss 1978. elements (Vrabec 1971; Smithson 1973; Schr6ter and W6bl Meyer et at. 1979; Galloway and Weiss 1981; Parker 1982) 1975; Mosher 1977) in order to validate the idea that field i showed how the motion of supergranules could redistribute moved to supergranulation boundaries by subsurface flors and concentrate magnetic flux tubes in and below the solar Good reviews of these observations may be found in Harm surface. Recent papers have confirmed and extended the earlier (1977) and Zwaan (1978, 1987). Recent observations of the work to include motions in granules (Schmidt et al. 1985) and motion of intranetwork fields at BBSO (Martin 1988) hmw have applied more elaborate two-dimensional and threeclearly shown motions of mixed polarity elements toward the dimensional computational techniques (Proctor and Weiss boundaries of the magnetic network. Further evidence Con1 1982; Galloway and Proctor 1983; Nordlund 1985a, b; Cattafrom Title, Tarbell and Topka (1987) who used high-resolutios neo 1984; Huriburt and Toomre 1988). The theoretical (0'5) data obtained at the Sacramento Peak Vacuum Towf analyses suggest that the observed magnetic and intensity Telescope to show that the loci of magnetic fields cmtre structures in the surface and higher layers of the solar atmobetter with velocity downdrafts and dark lanes in the granudsphere depend on the nature of both large-scale tion, than with upflows and bright regions in the contimum, Taken together, these observations suggest that not only tdi 'Operated by the Association of Univerities for Research in Astronomy. large-scale supergranular flow, but also the much smuNlA Inc-. under contract with the National Scienc Foundation. Partial support for motion fields of individual granules help to determine te the National Solar Observatory is provided by the USAF under a Memoranstructure of magnetic field on the Sun's surface. dum of Understanding with the NSF. The above discussion suggests that the flow determines dve I SOUP team includes L. Acton. D. Duncan. M. Finch. Z. Frank, G. Kelly. evolution and distribution of the magnetic fields. Howevr. It R. Lindgpe, M. Morrill. N. Osi (deceased . T. Pope. H. Ramsey. R. Reeves. R. Itehse. and R. Wallace. Lockheed Palo Alto Research Laboratory; J. Harvey. least in active regions, it is clear that the magnetic field Ires 1 J. Leibacber. W. Livingston, and L. November. National Solar Observatory. significant effect on the flow field. Zwaan (1978) has shown tha


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