Auroras are classified as discrete or diffuse, in accordance with the characteristics of their topside images. Diffuse auroras, even if structured according to ground-based imaging observations, lack the high intensity and shear phenomena exhibited by discrete auroras. Diffuse auroras have a variety of shapes ranging from linear to highly irregular, complex structures; the basic form of the discrete aurora is linear. The basic linear elements of discrete auroras undergo rotational development into vortex streets, the largest being spirals, ranging in size from 20 to 1,300 km. Spirals are clockwise, indicating upward current. Curls, of the order of 1 km in diameter, are counterclockwise. Folds are of intermediate size. Other deformations briefly discussed are flickering and splitting. Pulsating auroras occur in arc and arc-like forms and in patches of irregular shape. Black aurora is a lack of emission in a well-defined region within auroras which are intermediate between diffuse and discrete.