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Earth, Planets and Space
Fig. 6 | Earth, Planets and Space

Fig. 6

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

Fig. 6

Comparison of a ground-based and b satellite-based auroral images on December 7, 1999. The time sequence of selected auroral images is shown from top to bottom. All images are projected to the same area in geodetic coordinates. a Ground-based all-sky images (ASIs; 557.7 nm) at the Muonio station (MUO) in Finland. These ASIs were selected to show the observed instances a1 during the quiet interval, a2 at the start of Akasofu initial brightening (AIB), a3–a4 during AIB, a5 at the start of poleward expansion, and a6 during poleward expansion. b Global images (170 nm) taken by the Polar satellite ultraviolet imager (UVI). Each image was selected to form a pair with an ASI in a within 7 s. A comparison of a and b reveals that the longitudinally extended brightening (AIB) can be marginally observed in a2 and is evident in a3–a4 but not in b2–b4. In contrast, the brightening a5 that corresponds to the beginning of the poleward expansion was simultaneously observed in b5

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