Fig. 11

Synthesis of three different views of auroral substorm onset observations: a original concept (Akasofu 1964) based on 1-min resolution ground-based all-sky images (ASIs); b high time resolution (\(< \sim 10~\hbox {s}\)) ASIs; c satellite-based global images (resolution of a few minutes). The spatial resolution of ASIs (\(\sim 1~\hbox {km}\)) is much better than that of global images (\(\sim 50~\hbox {km}\)). From top to bottom, the time sequence of auroral emissions on the nightside ionosphere above \(60^\circ\) magnetic latitude is illustrated. The blue, green, and red colors indicate weak, moderate, and intense recorded auroral emissions, respectively. The initial brightening (IB) is longitudinally extended in a. This IB may appear as localized at the beginning followed by rapid longitudinal expansion (auroral rays or auroral beads) in b, as indicated by green circles. Red circles indicate poleward expansion (i.e., auroral breakup). A substorm onset is identified by the IB in a, and practically by the poleward expansion in c. It is undecided whether the localized IB or the poleward expansion should be used to define the substorm onset in b. Auroral brightness is significantly underemphasized in global images, presumably by area averaging when the aurora is latitudinally thinner than the spatial resolution of the images