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Solar rotation causes features on its surface to be viewed at different angles. The appearance of active region faculae changes dramatically with viewing angle. They have minimum contrast at disk center, but appear bright near the limb. The nature of this center-limb variation is related to the structure of the individual flux tubes comprising faculae. One class of models depict a facula as an evacuated flux tube with hot walls and a depressed cool floor (hot wall model). Another depicts them as hot clouds, because of internal heating. Both can explain the observed center-limb variation in contrast.
In this paper we ask whether there are other observable phenomena that can help differentiate between these models. The observations were obtained at the Swedish Solar Observatory, La Palma, using the Lockheed tunable filter. The data consist of co-registered images of line-of-sight magnetic field and of continuum intensity. The correlation between strong magnetic field and continuum intensity in active region faculae shows a remarkable and reproducible change between heliocentric angles of $50\deg$ and $60\deg$. These results support the hot wall model, but not the hot cloud model. Hybrid models in which the flux tube has properties of both models cannot by ruled out.
This work has been supported by NSF contract ATM-9320353, by NASA contracts NASW-4612 and NAS8-39747, and by Lockheed IR funds.
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