Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsp51a..13k&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SP51A-13
Physics
7511 Coronal Holes
Scientific paper
We investigate the physical properties of coronal holes using a histogram-based intensity thresholding technique. The method was tested on coronal holes observed by the SOHO/EIT and the Hinode/XRT instruments. The full-disk images were first transformed using the Carrington projection, which has the effect of removing off-limb pixels and preserving coronal hole areas at high latitudes. The maps were then decomposed into a series of overlapping sub-images from which histograms were extracted. The sub-images containing only quiet-sun regions showed a unimodal distribution, while those containing part of a coronal hole as well as quite Sun regions had bimodal intensity distributions. The latter distributions were modelled as the sum of two frequency distributions, representing contributions from quiet-sun and coronal hole pixels. The local minimum between the two intensity distributions was then used to identify the boundary between coronal hole and quiet-sun regions. This model was tested on a large number of EIT (19.5nm) and Hinode/XRT images obtained in Jan-Feb 2008. For both instruments, the local thresholding algorithm was found to successfully determine coronal hole boundaries in a consistent manner. This therefore enabled us to investigate the relationship between coronal holes and the solar wind at 1AU during this sample period.
Gallagher Peter. T.
Krista Larisza D.
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