Population-Alignment Collisional-Radiative Model

In this chapter, we construct the theoretical framework to deal with the Class 2 polarization. We assume a situation in which our plasma has an anisotropic EVDF (electron velocity distribution function). We assign a quantity the alignment to each level (more exactly, the ensemble of atoms (and ions) in this level) besides its number density or population. The presence of an alignment in a level makes emission lines originating from this level linearly polarized. The alignment is created by collisional excitation by electrons having an anisotropic EVDF. We construct rate equations for the alignment and for the population and solve them in the quasi-steady-state (QSS) approximation. We call this method the population-alignment collisional-radiative (PACR) model. We are thus able to interpret the observed polarization and intensity of emission lines in terms of the anisotropic EVDF.