Contrast of faculae at 1.6 microns

We followed Active Region NOAA 7981 from 1996 July 27 to 1996 August 7 at Big Bear Solar Observatory. During the region's limb-to-limb passage, images at 1.6 μm, 6103 Å, and CaK, as well as line-of-sight magnetograms were obtained every day to study the variation of facular/plage contrast and its relationship to magnetic fields. Our 1.6 μm images were observed by a high-quality 320 × 240 PtSi/Si detector, which produces extremely uniform images. Our data agree with the early results of Foukal et al. in several aspects: (1) at 1.6 μm, some faculae are dark at solar disk center and all become bright when they are close to the limb; (2) the changeover occurs approximately at cos θ = 0.5-0.7; (3) the threshold of magnetic flux required to produce a dark structure at 1.6 μm is about 5 × 10¹⁸ Mx. Equally important, our result is different from that of Foukal et al. on an important issue: among about 150 elements studied near the disk center, only four of these IR dark faculae show no contrast in the visible continuum. Other elements show dark contrasts in both 1.6 μm and the visible continuum, if there is sufficient resolution in the data. However, darkening of weaker (lower flux) elements are more obvious at IR and most bright points seen at red continuum disappear at IR. These findings do confirm that 1.6 μm images reveal new aspects of photospheric magnetic structure. In this paper, we also present a quantitative relationship between the dark contrast of 1.6 μm faculae and magnetic field strengths at the disk center, as well as the contrast variation of IR faculae as a function of the disk position.