Facular Structures Derived from Precise Two-Color Contrast Observations

A precise solar surface photometric observing system was developed and intensity observations of the whole Sun were carried out at a fairly quiet period from December 1987 to April 1988, attaining the photometric accuracy of ≈ 10−3. Using 28 days observation of 4 hours duration each and 0.3 s time interval, we obtained the center-to-limb variation of the ratio of the facular intensity contrast (ΔIf÷Io) between two colors of 545 nm (G) and 770 nm (R): (ΔIf÷I0)G÷(ΔIf÷I0)R = 2.20 − 1.16μ, where μ is the direction cosine between the line of sight and the surface normal. While this relationship was obtained mostly for active regions from the whole Sun data, we also found an almost identical relation for the quiet Sun network by excluding active regions. This suggests the similarity of faculae at both places.