A classical theory of coronal emission line polarization

We present a classical theory of formation for polarized, magnetic dipole e mission lines in the magnetized solar corona. Because of the small Einstein A-coefficients of forbidden lines and the expected magnetic field strength s in the corona, coherences between different magnetic substates can be neg lected, so the observed Stokes vector for these lines is the result of the incoherent superposition of the Stokes vectors emitted in the de-excitation of the individual magnetic substates of the transition's upper level. Usin g classical electrodynamics and a weak-field expansion formalism, we could derive the main polarization properties of the transition J = 1 to J = 0, i n the collisionless regime. In particular, we derived the correct amount of atomic alignment in the upper level, induced by the anisotropic, unpolariz ed illumination from the photosphere, and the dependence of Stokes Q and U linear polarization on the magnetic held direction in the plane of the sky. The influence of atomic alignment on the V profile is also correctly repro duced. This work provides a classical interpretation of the physical proces s that generates atomic alignment in the radiating ion and how the associat ed Van Vleck effect in resonance-scattering linear polarization and the ali gnment contribution to Zeeman effect circular polarization come about.