G-band spectral synthesis in solar magnetic concentrations

Narrowband imaging in the G band is commonly used to trace the small magnetic 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 contrast referring to the quiet Sun is about twice the contrast in continuum wavelengths. The agreement with observations does not depend on the specificities of the model atmosphere; rather, it holds from single flux tubes to microstructured magnetic atmospheres. However, the agreement requires that the real G-band bright points are not spatially resolved, even in the best observations. Since the predicted G-band intensities exceed by far the observed values, we foresee a notable increase of contrast of the G-band images upon improvement of the angular resolution. According to the LTE modeling, the G-band spectrum emerges from the deep photosphere that produces the continuum. Our syntheses also predict solar magnetic concentrations showing up in continuum images but not in the G band. Finally, we have examined the importance of the CH photodissociation in setting the amount of G-band absorption. It turns out to play a minor role.