G‐Band Spectral Synthesis in Solar Magnetic Concentrations

Narrow band imaging in the G-band is commonly used to trace the smallmagnetic field concentrations of the Sun, although the mechanism that makesthem bright has remained unclear. We carry out LTE syntheses of the G-band inan assorted set of semi-empirical model magnetic concentrations. The synthesesinclude all CH lines as well as the main atomic lines within the band-pass. Themodel atmospheres produce bright G-band spectra having many properties incommon with the observed G-band bright points. In particular, the contrastreferred to the quiet Sun is about twice the contrast in continuum wavelengths.The agreement with observations does not depend on the specificities of themodel atmosphere, rather it holds from single fluxtubes to MIcro-StructuredMagnetic Atmospheres. However, the agreement requires that the real G-bandbright points are not spatially resolved, even in the best observations. Sincethe predicted G-band intensities exceed by far the observed values, we foreseea notable increase of contrast of the G-band images upon improvement of theangular resolution. According to the LTE modeling, the G-band spectrum emergesfrom the deep photosphere that produces the continuum. Our syntheses alsopredict solar magnetic concentrations showing up in continuum images but not inthe G-band . Finally, we have examined the importance of the CHphoto-dissociation in setting the amount of G-band absorption. It turns out toplay a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure