On the Relation Between Facular Bright Points and the Magnetic Field

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Session 86 -- Solar Active Regions
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[86.07] On the Relation Between Facular Bright Points and the Magnetic Field

Thomas Berger (Stanford University), Richard Shine (LPARL), Theodore Tarbell (LPARL), Alan Title (LPARL), G\"oran Scharmer (Royal Academy of Sweden)

Multi-spectral images of magnetic structures in the solar photosphere are presented. The images were obtained in the summers of 1993 and 1994 at the Swedish Solar Telescope on La Palma using the tunable birefringent Solar Optical Univeral Polarimeter (SOUP~filter), a 10~\AA\ wide interference filter tuned to 4304~\AA\ in the band head of the CH radical (the Fraunhofer G-band), and a 3~\AA\ wide interference filter centered on the Ca~II--K absorption line. Three large format CCD cameras with shuttered exposures on the order of 10~msec and frame rates of up to 7 frames per second were used to create time series of both quiet and active region evolution. The full field--of--view is $60\times 80$ arcseconds ($44\times 58$~Mm). With the best seeing, structures as small as 0.22 arcseconds (160~km) in diameter are clearly resolved. Post--processing of the images results in rigid coalignment of the image sets to an accuracy comparable to the spatial resolution. Facular bright points with mean diameters of 0.35 arcseconds (250~km) and elongated filaments with lengths on the order of arcseconds ($10^3$~km) are imaged with contrast values of up to 60~\% by the G--band filter. Overlay of these images on contemporal Fe~I 6302~\AA\ magnetograms and Ca~II~K images reveals that the bright points occur, without exception, on sites of magnetic flux through the photosphere. However, instances of concentrated and diffuse magnetic flux and Ca~II~K emission without associated bright points are common, leading to the conclusion that the presence of magnetic flux is a necessary but not sufficient condition for the occurence of resolvable facular bright points. Comparison of the G--band and continuum images shows a complex relation between structures in the two bandwidths: bright points exceeding 350~km in extent correspond to distinct bright structures in the continuum; smaller bright points show no clear relation to continuum structures. Size and contrast statistical cross--comparisons compiled from measurements of over two-thousand bright point structures are presented. Preliminary analysis of the time evolution of bright points in the G--band reveals that the dominant mode of bright point evolution is fission of larger structures into smaller ones and fusion of small structures into conglomerate structures. The characteristic time scale for the fission/fusion process is on the order of minutes.

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