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Photometric measurements of the continuum contrast of active region plage, after carefully removing all sunspots and pores, has been obtained for several years from the Swedish Solar Observatory, La Palma. More recently, similar measurements have been obtained for magnetic elements in the network far from active regions. These data were taken with the Lockheed tunable filter instrument using a CCD camera.
The measurements indicate that at disk center active region plage is dark (negative contrast: intensity less than the surrounding quiet sun), while the network is bright. Furthermore, active region plage rapidly turns bright toward the limb; the network also brightens but more slowly. We have constructed a simple model that assumes that the magnetic lines-of-force are approximately vertical in active region plage, while those in the quiet sun network are inclined at the photosphere. This model correctly predicts the continuum contrast of network at disk center, and also its variation from center-to-limb. This provides evidence that the network is often inclined from vertical by 30 degrees or more.
This work was supported by Lockheed IR Funds, by NASA contracts NAS8-32805 (SOUP), NAS5-26813 (OSL), NAS5-30386 (MDI), and NAS8-38106 (BSOUP), and NSF contract ATM-8912841.
