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 passa ge, images at 1.6 mu m, 6103 Angstrom and CaK, as well as line-of-sigh t magnetograms were obtained every day to study the variation of facul ar/plage contrast and its relationship to magnetic fields. Our 1.6 mu m images were observed by a high-quality 320 x 240 PtSi/Si detector, w hich produces extremely uniform images. Our data agree with the early results of Foukal et al. in several aspects: (1) at 1.6 mu m, some fac ulae are dark at solar disk center and all become bright when they are close to the limb; (2) the changeover occurs approximately at cos the ta = 0.5-0.7; (3) the threshold of magnetic flux required to produce a dark structure at 1.6 mu m is about 5 x 10(18) Mr. Equally important, our result is different from that of Foukal et al. on an important is sue: among about 150 elements studied near the disk center, only four of these IR dark faculae show no contrast in the visible continuum. Ot her elements show dark contrasts in both 1.6 mu m and the visible cont inuum, if there is sufficient resolution in the data. However, darkeni ng of weaker (lower flux) elements are more obvious at IR and most bri ght points seen at red continuum disappear at IR. These findings do co nfirm that 1.6 mu m images reveal new aspects of photospheric magnetic structure. In this paper, we also present a quantitative relationship between the dark contrast of 1.6 mu m faculae and magnetic field stre ngths at the disk center, as well as the contrast variation of IR facu lae as a function of the disk position.