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Earth, Planets and Space
Fig. 8 | Earth, Planets and Space

Fig. 8

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

Fig. 8

Solar wind parameters and geomagnetic indices on December 7, 1999. These data were obtained from the Operating Missions as Nodes on the Internet (OMNI) data set. The red vertical lines indicate the Akasofu initial brightening (AIB; 2124:50 UT) and the poleward expansion (2127:50 UT) identified by using all-sky images (ASIs). The solar wind parameters were time-shifted with respect to the bow shock nose. Geocentric solar magnetospheric (GSM) coordinates were used. The interplanetary magnetic field (IMF) was weakly southward (\(B_{z}~\sim ~-1~\hbox {nT}\)) and strongly duskward (\(B_{y}~\sim ~6~\hbox {nT}\)), indicating moderately favorable conditions for the occurrence of substorms. The AL index (i.e., westward ionospheric current) began to develop at 2129 UT, which is closer in time to the poleward expansion than to the AIB in the ASIs

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