Theoretical study of the polarization by resonance scattering of the Hα line in spicules

One currently observes spicules in the H alpha line, the 1000 underlying photospheric radiation being scattered by the matter of the spicules. In this paper we solve the equations of statistical equilibrium for a hydrogen atom having an ensemble velocity and being illuminated by the photospheric radiation field, This leads us to the determination of the Stokes parameters I, Q, U of the radiation diffused by the spicule towards the observer. The calculations include fine structure effects, limb darkening and depolarization by isotropic collisions. One obtains the linear polarization degree as well as the rotation of the polarization plane as a function of the ascending velocity of matter in spicules.