Observed microstructure of auroral forms

An inherent characteristic of most auroral forms is their highly structured nature. No general explanation for the structure exists in spite of much efforts. Auroral shape, development from one shape to another, spiral motion and temporal variation have been studied. The basic form of discrete aurora is linear, whereas diffuse aurora shows a variety of shapes ranging from linear to highly irregular, complex structures. Dynamical variation within discrete aurora is characterized by shear, eventually caused by field-aligned current and charge excess. In contrast, diffuse, pulsating aurora exhibits little shear, indicating lack of large current and electric field associated with this type of aurora. (Yoshimori, M.)