Facular structures derived from precise two-color contrast observations

Abstract

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 (ΔI f÷Io) between two colors of 545 nm (G) and 770 nm (R): (ΔI f÷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. Using the above relation and the difference of optical depth, τ, in two colors, we found that the facular temperature gradient, dT/dτ G, is smaller than that of the quiet photosphere if the gradient is measured more or less vertically (i.e., seen at μ > 0.7) in accord with Foukal and Duvall (1985), while it is larger than that of the quiet photosphere if the temperature gradient is measured more obliquely (i.e., seen at μ > 0.7). These findings are free from the low spatial resolution of the present observations because the contrast ratio was used, and also independent of a specific model of hot-wall or hillock. In particular, if the true contrast of facular bright points of 0′'2 size is taken as 40% in the green from high-resolution observations, the facular point temperature may even be increasing towards higher geometrical levels ar ▿ G ≈ 1. We found also that the area filling factor in active regions is on the order of a few percent or so in accordance with the previous studies.