A Three-Dimensional Magnetohydrodynamic Model of Planetary Nebula Jets,   Knots, and Filaments
  The morphologies of planetary nebulae are believed to be self-organized configurations. These configurations are modeled by three-dimensional temporally self-similar magnetohydrodynamic solutions with radial flow, under the gravitational field of a central star of mass $M$. These solutions reproduce basic features, such as jets, point-symmetric knots, and filaments, through plasma pressure, mass density, and magnetic field lines. The time evolution function of the radial velocity starts as a slow wind and terminates as a fast wind. 
K.H. Tsui