Fig. 18From: Auroral substorms as an electrical discharge phenomenon a A part of the UL circuit which produces a positive Bz field (red arrow) which is mainly responsible for the poleward expansion of auroras and the electrojet, b a series of all-sky photographs (an example of the poleward expansion), c an expanded aurora system, and also the extended auroral electrojet (Courtesy of Y. Kamide)Back to article page