Intense growth phase events of substorms

Over the past few decades, several events of the intense growth phase preceding the expansion onset of substorms were reported. We reexamine these events using an extensive set of data, including ground-based magnetograms at auroral latitudes, solar wind data from the Wind and IMP 8 satellites, and auroral UV images from the Polar and Viking satellites. Magnetograms show that the intense auroral electrojet is located in the early morning sector during these events. A common feature among the corresponding solar wind conditions for these events is the existence of a period of southward interplanetary magnetic field (IMF) preceded by a prolonged northward IMF. This intense growth phase, however, seems to be independent of the IMF B_y polarity. Enhanced auroras in the early morning sector appear during the growth phase. These auroras are consistent with relatively large geomagnetic disturbances during these events, indicating that enhanced conductances, combined with enhanced electric field caused by southward IMF, play an important role in the auroral electrojet during these intense growth-phase events.