Proxy magnetometry of the photosphere: why are G-band bright points so bright?

Abstract

We discuss the formation of G-band bright points in terms of standard uxtube modeling, in particular the 1D LTE models constructed by Solanki and coworkers. Combined with LTE spectral synthesis they explain observed G-band bright point contrasts quite well. The G-band contrast increase over the continuum is due to the enhancement of the hot- wall effect by CH depletion within the uxtube. The modeling predicts that the CN band at 388 nm shows magnetic elements at yet higher contrast, which was tested using the last light of the Swedish Vacuum Solar Telescope on La Palma. The issue whether the standard uxtube models have erroneous temperature stratifications due to neglect of NLTE irradiation remains open. It is important because the stratifications imply considerable heating.