A model for the formation of auroral arcs is presented which provides an explanation for the extraordinary small thickness of auroral arcs and other morphological properties. The presence of field-aligned current sheets allows for patchy (three-dimensional) reconnection processes. Due to the presence of magnetic shear, the associated field-aligned electric fields map to thin elongated regions at low altitudes. The ionospheric foot points of newly reconnected field lines can be interpreted as the locations of the auroral display. The process can occur in a casacading manner at varying altitudes depending on the local current densities and plasma conditions. The proposed properties are confirmed by results of three-dimensional magnetohydrodynamic computations. ¿ American Geophysical Union 1993