  Spectral lines of helium are commonly observed on the Sun. These observations
contain important informations about physical conditions and He/H abundance
variations within solar outer structures. The modeling of chromospheric and
coronal loop-like structures visible in hydrogen and helium lines requires the
use of appropriate diagnostic tools based on NLTE radiative tranfer in
cylindrical geometry. We use iterative numerical methods to solve the equations
of NLTE radiative transfer and statistical equilibrium of atomic level
populations. These equations are solved alternatively for the hydrogen and
helium atoms, using cylindrical coordinates and prescribed solar incident
radiation. Electron density is determined by the ionization equilibria of both
atoms. Two-dimension effects are included. The mechanisms of formation of the
principal helium lines are analyzed and the sources of emission inside the
cylinder are located. The variations of spectral line intensities with
temperature, pressure, and helium abundance, are studied. The simultaneous
computation of hydrogen and helium lines, performed by the new numerical code,
allows the construction of loop models including an extended range of
temperatures.
