Properties of the smallest solar magnetic elements .2. Observations versus hot wall models of faculae

Observations obtained at the Swedish Solar Observatory, La Palma, using the Lockheed tunable filter, have been used to measure properties of active region faculae, including contrast from disk center to near the limb. The data consist of coregistered digital photometric images of the line-of-sight magnetic field and of the continuum intensity. The results are related to the structure of the individual flux tubes comprising faculae in active regions. In addition to center-limb contrast, the observations reveal a change in contrast between heliocentric angles of about 45 degrees and 60 degrees related to the ''turn-on'' of bright faculae. A class of models has been constructed that describes a facula as an evacuated thin flux tube with a hot wall and a depressed cool floor (hot wall model). The hot wall model is very successful in predicting the observations, including the changes observed between 45 degrees and 60 degrees. The model predicts that the larger flux tubes comprising active region plage are micropores with a Wilson depression of 100 km largely independent of micropore diameter. Their typical diameter is 350-650 km; the largest ones are about 1200 km across. Bright points are the major component of active region plages by number, but micropores probably are the major component by total magnetic flux. Bright active region faculae seen near the limb are simply micropores viewed from the side, where the hot wall is visible and the depressed cool floor is not. The temperature difference between cool floor and hot wall varies from 300 to nearly 500 K, depending on tube diameter and heliocentric angle. These results have important implications for solar irradiance variations.