The development of discrete active auroral forms

Using analytical and numerical methods, it is shown that all the basic forms of the active arcs can be described within the framework of a nonlinear Alfven wave model. The auroral forms are then considered as nonlinear two-dimensional Alfven vortex structures that are uniform along the ambient magnetic field. They can exist in a cold inhomogeneous plasma, e.g., the Earth´s ionosphere. Large-scale spirals and folds can recover the initial form of a typical arc. In the case of a small solitary disturbance of the current density or a sharp bending of the current sheets, vortex chains are formed in the spirals and the folds