Mathematics – Logic
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........14b&link_type=abstract
Thesis (PHD). STANFORD UNIVERSITY , Source DAI-B 57/11, p. 6984, May 1997, 270 pages.
Mathematics
Logic
4
Photosphere
Scientific paper
Properties of small-scale magnetic structures in the photosphere are analyzed in multi-spectral time-series image sets obtained at the 50 cm Swedish Solar Vacuum Telescope (SVST) on the island of La Palma, Spain. Several of the images are among the highest resolution images of the solar photosphere yet obtained. Sub-arcsecond-scale magnetic 'elements' are identified, segmented, and tracked using bright points found in very high spatial resolution G-band 4305 A filtergrams. Simultaneous images including Ca II K-line filtergrams, Fe I 6302 A magnetograms, and 4686 A broadband continuum filtergrams allow cross-wavelength comparison of properties. Angular resolution of the filtergrams is typically 0.25 '' and temporal resolution is in the range of 20-100 sec; magnetogram resolution approaches 0.3'' in some images and is generally below 0.5''. To above an 84% statistical confidence level, G-band bright points occur exclusively at sites of kilogauss, sub-arcsecond, magnetic flux concentrations in the photosphere; magnetic flux concentration is a necessary but not sufficient condition for the occurrence of G-band bright points. The measured distribution of magnetic element diameters in active region network is log-normal with a modal value of 220 km (0.3''). The smallest elements observed are 120 km (0.17'') in diameter; the largest are about 600 km (0.7'') in diameter. The average contrast with respect to quiet Sun of magnetic elements in the G-band is 30%: 2-3 times higher than the average continuum contrast. Magnetic element contrast does not vary with size within the size range of G-band bright point measurements. Average contrast increases with limbward heliocentric angle to a peak of about 80% at μ = /cosθ = 0.3; there is evidence of a decrease with further increase in angle. Magnetic elements undergo a continual fragmentation/merging evolution driven by the granular convective flowfield of the photosphere; morphological time scales are on the order of 100 seconds. Velocities of individual elements range from 1-5 km s-1 with an RMS value of 2.4 km s-1. The range of motion is typically on granular and mesogranular scales (1000-2500 km) with an average value of 2100 km. Individual fragments from clusters have a characteristic lifetime on the order of the granulation correlation time (6-8 minutes). The lifetime of clusters associated with persistent sinks in the granular flowfield is on the order of hours. Classical statistical analysis of displacement versus time yields a diffusion coefficient for network magnetic elements of 224.8± 0.2 km s-1. In general, the results are inconsistent with the idea of small-scale magnetic flux in the photosphere being contained in stable, isolated, 'flux tubes' and emphasize the need for better understanding of the formation and the thermal (and∨ non-thermal) heating of magnetic regions in the photosphere.
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