Multi-color photometric observations of facular contrasts

Abstract

We have analyzed high-resolution, digital, photometric images of solar active regions made at various center-limb positions on 21–24 July, 1983. The images were made at three continuum wavelengths: 5245 Å in the green (bandpass 1.5 Å), 6264 Å in the red (bandpass 1.5 Å), 10 000 Å in the infrared (bandpass 3 Å), and also at 8662 Å in the Caii infrared line (bandpass 3 Å). In all continuum colors, the contrasts of facular patches, as opposed to individual facular elements, appear to behave as linear functions of 1/cos θ, where θ is the heliocentric angle (μ = 0 at the limb, 1 at disk center). The relative contrasts in the different continuum colors are roughly proportional to (wavelength)^-1, as expected from a Planck distributioin in the Wien approximation. The observed variation of the relative contrasts with center-limb position is compared to two simple theoretical models.