Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt.........9l&link_type=abstract
Thesis (PhD). UNIVERSITY OF COLORADO AT BOULDER, Source DAI-B 65/06, p. 2968, Dec 2004, 210 pages.
Other
4
Scientific paper
The sun exhibits turbulent convection on multiple spatial and temporal scales. We assess the properties of supergranulation (the largest readily observed convection scale) and its interaction with other scales of motion in the solar convection zone. The Michelson Doppler Imager (MDI) aboard the SOHO spacecraft provides Doppler and magnetogram full-disk images at rapid (1 min) cadence, undistorted by Earth's atmosphere and uninterrupted by a day-night cycle. This permits tracking specific solar regions for continuous study over intervals as long as 8 days with a spatial resolution previously unattainable. We have applied the local correlation tracking (LCT) algorithm to full-disk MDI data from the years 1999 and 2001 in order to study near-surface solar flows. The LCT algorithm exhibits a disk-centered convergence artifact due to unresolved granulation. We discuss the cause and provide possible solutions to this problem. Once the artifact is ameliorated, we can detect large-scale flows comparable to solar subsurface weather reported from helioseismic studies. We find that supergranulation is characterized by outflow sites surrounded by narrower convergence lanes, with these cellular patterns filling the solar surface. We are able to identify and track approximately 40,000 supergranules as they merge, fragment, and evolve over their lifetimes. Small-scale magnetic elements appear to be passively advected from the interiors of supergranules to convergence lanes at their periphery, often to be collected at the network interstices. We confirm that the average supergranule lifetime is approximately 25 hr, yet we have also detected some very long-lived supergranules whose lifetimes may exceed 8 days. We have discovered a weak but persistent alignment of solar supergranulation in the direction parallel to the solar rotation axis. The signature of this anisotropy is observed in multi-day temporal averages as well as in spatial and temporal spectral analyses. By studying the temporal duration, pattern propagation speed, and spatial modulation of the supergranular alignment, we find evidence for the existence of underlying giant cells of convection.
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