Radiative transfer in cylindrical threads with incident radiation. II. 2D azimuth-dependent case

A method is proposed for the solution of NLTE radiative transfer equations in long cylinders with an external incident radiation that varies with direction. This method is designed principally for the modelling of elongated structures imbedded in the solar corona (loops, prominence threads). The radiative transfer problem under consideration is a 2D one, since the source functions and absorption coefficients vary with both distance to axis and azimuth. The method is based on the general principles of finite-differences and accelerated Lambda -iteration. A Fourier series is used for interpolation in azimuth. The method is applied to a line emitted by a two-level atom with complete frequency redistribution. Convergence properties of the method and influence of the inclination angle on the source function are discussed.