Fig. 6From: Auroral substorms as an electrical discharge phenomenonThe UL current is the ionospheric part of Bostrom’s 3-D current circuit during the expansion phase [mainly the Hall current] (Bostrom. 1964). It can be driven by an earthward-directed electric field on the equatorial plane where E J < 0. Note that in the ionosphere, E J p > 0Back to article page