Precise wideband photometry of photospheric faculae with an emphasis on the disk center

Abstract

The center-to-limb variation of the excess intensity in faculae was obtained for 266 active regions with an accuracy of ≦10⁻³. For this observation full-disk images were obtained with a rotating one-dimensional diode array whose rotation axis was set at the disk center, at the wavelength of 5450 Å with a bandpass of 400 Å. From the center-to-limb variation of excess intensity of active regions the excess effective temperature was found to be 6.4 K on the average where the mean longitudinal magnetic field is 65 G as measured by 5233 Å line. In other words the ratio of the excess radiative flux to the total flux was 0.44% on the average for the present measurements of low spatial resolution of 20″.

The average excess intensity for 60 active regions near the disk center was found to be 4 × 10⁻⁴ of the quiet Sun intensity. This very low excess brightness averaged over the whole active region, in contrast to the reported high excess brightness of facular points (diameter ≈0.2″) of ≈0.4, leads to a hypothesis that the background in between facular points in the active region is darker than the true quiet photosphere by ≈1%. It is further surmised that the inferred darkness of intra facular points is due to partial compensation for excess total irradiance of facular points. This interpretation is also consistent with previous observations of the contrast of facular points near the limb.