Discriminant Analysis of Bright Points and Faculae: Center-to-Limb Distribution, Contrast and Morphology

Details Discriminant Analysis of Bright Points and Faculae: Center-to-Limb Distribution, Contrast and Morphology Discriminant Analysis of Bright Points and Faculae: Center-to-Limb Distribution, Contrast and Morphology

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008espm...12.2.60k&link_type=abstract

"12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/,

Physics

Scientific paper

High-resolution images of the solar photosphere reveal an intriguing mixture of Brights Points (BPs) and faculae at several disk positions, which is not explained by the conventional "hot wall'' model. Together with quantitative discrepancies between observations and simulations of faculae, it stresses that the fundamental relationship between BPs and faculae is not yet clear: How are BPs and faculae distributed on the solar disk? How do the photometric properties of BPs and faculae differ and vary with disk position?
To tackle these issues, a necessary step is to sort the BPs and faculae at various disk positions, in order to treat them separately. We present here the first attempt to discriminate BPs and faculae, using a statistical classification approach based on Linear Discriminant Analysis (LDA). This has never been done so far, presumably due to the lack of known automated methods to distinguish such features, and to the difficulty to obtain a coherent dataset of high-resolution images recorded in the same conditions. We applied our method to high-resolution G-band and continuum images of active regions recorded at the Swedish Solar Telescope, covering several disk positions where the transition from BPs to faculae is expected.
This allowed us to retrieve a first estimate of the center-to-limb variation of the relative distribution of both species. The center-to-limb distribution of BPs and faculae reveals the predominance of faculae at all disk positions except close to disk center. We argue that these ubiquitous faculae could be the transient signatures of swaying flux tubes with a wide range of inclination angles. Moreover, we statistically compared the G-band and continuum contrast of BPs and faculae, and characterized their morphology. Both the G-band and continuum contrast of BPs and faculae are found to similarly increase from center to limb. But when comparing G-band to continuum, BPs and faculae exhibit slightly different behaviours, which are related to radiative transfer processes. By orienting the features in local coordinate frames corresponding to the principal axes of their contrast moment of inertia, we could retrieve characteristic G-band contrast profiles exhibiting the typical predicted asymmetry for faculae. Finally, our BPs and faculae were found to have very similar morphological properties.
Although our study is essentially descriptive and based on purely photometric information, we hope that it will provide novel useful constraints for future BPs/faculae MHD models.

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