G-band spectral synthesis in solar magnetic concentrations

Narrowband imaging in the G band is commonly used to trace the small magnet ic field concentrations of the Sun, although the mechanism that makes them bright has remained unclear. We carry out LTE syntheses of the G band in an assorted set of semiempirical model magnetic concentrations. The syntheses include all CH lines as well as the main atomic lines within the bandpass. The model atmospheres produce bright G-band spectra having many properties in common with the observed G-band bright points. In particular, the contr ast referring to the quiet Sun is about twice the contrast in continuum wav elengths. The agreement with observations does not depend on the specificit ies of the model atmosphere; rather, it holds from single flux tubes to mic rostructured magnetic atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best obs ervations. Since the predicted G-band intensities exceed by far the observe d values, we foresee a notable increase of contrast of the G-band images up on improvement of the angular resolution. According to the LTE modeling, th e G-band spectrum emerges from the deep photosphere that produces the conti nuum. Our syntheses also predict solar magnetic concentrations showing up i n continuum images but not in the G band. Finally, we have examined the imp ortance of the CH photodissociation in setting the amount of G-band absorpt ion. It turns out to play a minor role.