The formation of G-band bright points I: Standard LTE modelling

Assuming LTE, we synthesise solar G band spectra from the semiempirical flux-tube model of Briand & Solanki (1995). The results agree with observed G-band bright-point contrasts within the uncertainty set by the amount of scattered light. We find that it is the weakening of spectral lines within the flux tube that makes the bright-point contrast in the G band exceed the continuum contrast. We also synthesise flux-tube spectra assuming LTE for the full wavelength range from UV to IR, and identify other promising passbands for flux-tube observations. 1. Spectrum synthesis of G-band bright points Filtergrams taken in the G-band around 430.5 nm are often used for proxy magnetometry since photospheric bright points associated with magnetic structures show up very well in them (e.g. Berger et al. 1995). We have performed LTE spectrum synthesis of the G band (mostly formed by CH) using the semiempirical magnetic flux-tube model NCHROM7 of Briand & Solanki (1995) which is a refinement of the models of Bruls & Solanki (1993) and Solanki (1986). It is embedded in a standard model for the quiet photosphere. Figure 1 shows results. The hot bright walls of the (rotationally symmetric) flux tube create a bright ring seen as a double peak in the radiation temperature. Figure 1 also shows the temperature structure along three lines of sight: through quiet sun, through the tube centre, and through the tube wall where the emergent intensity peaks considerably because it samples the quiet-sun model at larger depth than the quiet-sun line of sight does. The peak contrast with respect to the quiet sun is larger in G than in C mainly because the photosphere appears darker in the G band. Detailed inspection of the synthetic spectra shows how most spectral lines weaken in the flux tube. We interpret this as due to the shallower temperature