In this paper, we examine the evolutionary process of expansion of a magnetized ring of cool matter ejected from a Red Giant. The subsequent formation of a circumstellar shell or preplanetary nebula is also analyzed. The velocity, density and magnetic field are calculated along a time-sequence, on a large two-dimensional grid of points close to and far away from the star. Owing to long range forces such as thermal pressure gradients and magnetic stresses, the velocity field becomes a linear function of the distance. The latitudinal dependence of this function is poor, while conversely the density distribution is clearly aspheric. The magnetic field intensity at a distance 1015 - 1016 cm is in good agreement with the observational data (Reid et al. 1979, Claussen and Fix 1982, Chapman and Cohen 1986, Cohen 1987). The isophotes are plotted and compared to observational contours obtained from the literature. The filamentary structure appearance and its stability within the envelope are also discussed.