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Fig. 1 | Earth, Planets and Space

Fig. 1

From: Simultaneous observation of auroral substorm onset in Polar satellite global images and ground-based all-sky images

Fig. 1

Original illustrations and figure captions of the Akasofu substorm onset (Akasofu 1964). Reprinted with permission from Elsevier. Clarifications are added on the bottom of the figure. The illustrated time sequence was proposed on the basis of 1-min resolution ground all-sky images (ASIs). Auroral emissions in the polar ionosphere above \(60^\circ\) magnetic latitude are illustrated. \(T = 0\) min represents the time of the Akasofu substorm onset. a \({T} < 0\): Quiet time. Quiet-time auroral arcs are shown. b T = 0 ~ 5 min: Akasofu initial brightening (AIB; i.e., Akasofu substorm onset), starting at \(T = 0\). Also called Stage 1 of the substorm expansion phase. This brightening is wide in longitude without poleward expansion. c \(T = 5 \sim 10\) min: Poleward expansion, starting at \(T = 5\). Also called Stage 2 of the substorm expansion phase. The two-stage development has also been illustrated in later studies (e.g., Akasofu et al. 2010) and is essential in this Akasofu substorm onset

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