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SPRG Seminars - Archive
March 7, 2006:
High resolution observations of solar faculae
Thomas Berger, Lockheed Martin Solar and Astrophysics Lab
Solar faculae are the bright, small-scale, elements typically seen around sunspot active regions near the limb. Faculae are known to be associated with magnetic field concentrations outside of sunspots and are the main contributor to the approximately 0.1% increase in total solar irradiance at the peak of the solar activity cycle. Accurately modeling solar irradiance as a function of magnetic activity on the star is an important aspect of predicting solar irradiance effects on terrestrial climate. However due to the challenge of resolving the small-scale structure of faculae, facular irradiance is typically modeled using empirical contrast functions derived from low resolution images and magnetograms. Recent observations from the Swedish 1-meter Solar Telescope on La Palma have achieved unprecedented 100 km spatial resolution images and magnetograms of faculae. The new data reveal surprising details of the magnetoconvective nature of faculae and produce a function of irradiance versus magnetic flux that is distinctly different than those produced using previous lower resolution data: facular contrast is approximately constant as a function of magnetic flux density. Faculae are also shown to be bright out to the extreme limb in contradiction to earlier studies that implied a sharp decrease in facular brightness near the limb. The main conclusion is that faculae are granules made relatively bright by the opacity reduction along lines of sight that traverse magnetic flux concentrations. Recent 3D compressible MHD models of faculae verify this conclusion and point to a need to replace the empirical irradiance models with physical models incorporating realistic radiative transfer.