The Source Function in a Non-Equilibrium Atmosphere. VIII. Solution for a Two Level Atom in a Finite Atmosphere.

The solution for the variation with optical depth of the source function for a two-level atom in a finite atmosphere, with and without temperature gradient, is presented. Even for an optical thickness of the atmosphere as high as tO , the emergent intensity differs by an order of magnitude from that for the semi-infinite case. For the case of non-symmetric temperature gradient, the asymmetry in source function is very considerably less than for B (T ). The use of a model consisting of a finite slab isolated in space is a better approximation for such high-excitation ions as He ii, in a low-excitation atmosphere such as the sun, than is the model of a semi-infinite atmosphere.