Resonance-line polarization. Pt. 2

Resonance-line polarization in a spherically symmetric, plane-parallel atmosphere has been explored by making numerical calculations of the center-to-limb variation of the intensity and polarization line profiles for the two solar UV emission lines Si IV 139.38 nm and C III 97.70 nm. The model of the line-forming layer in the chromosphere-corona transition region has been specified by three parameters: the temperature gradient, the electron pressure, and the line-broadening velocity. As collisional depolarization is insignificant in the transition region, the polarization turns out to depend on the model atmosphere only through tausub(d), the optical thickness of the line-forming layer, regardless of what combination of the three atmospheric parameters has generated a given value of tausub(d). Optically thin radiation is 'thermal' and therefore unpolarized. Multiple scattering enhances the polarization, but as the anisotropy in the radiation field (limb-brightening) decreases with increased optical thickness, the initial rise in the polarization will be followed by a decline as tausub(d) increases. When tausub(d) is small, the polarization is constant throughout the core of the line but decreases in the far wings due to the increased contribution from the unpolarized continuum. For large values of tausub(d) the polarization is reduced in the opaque line core, and maxima develop in the wings where the line-forming layer becomes transparent. (orig.)