In this paper, we examine the evolutionary process of expansion of a m
agnetized ring of cool matter ejected from a red giant. The subsequent
formation of a circumstellar shell or preplanetary nebula is also ana
lyzed. The velocity, density, and magnetic field are calculated along
a time sequence, on a large two-dimensional grid of points close to an
d far away from the star. Owing to long-range forces such as thermal p
ressure gradients and magnetic stresses, the velocity field becomes a
linear function of the distance. The latitudinal dependence of this fu
nction is poor, while conversely the density distribution is clearly a
spheric. The magnetic field intensity at a distance approximately 10(1
5)-10(16) cm is in good agreement with the observational data. The iso
photes are plotted and compared to observational contours obtained fro
m the literature. The filamentary structure appearance and its stabili
ty within the envelope are also discussed.