Strong Substorm Development From Polar-Cap Arc Laydown Along the Auroral Poleward Boundary

Meso-scale enhanced flow channels often extend across the polar cap from the dayside to the nightside auroral oval, where they enter the oval via localized reconnection and cause auroral oval disturbances. Examples here show such flows associated with azimuthally moving polar cap arcs that develop an equatorward portion that bends in the direction opposite to the arc's motion, becoming nearly parallel to the nightside auroral oval and possibly becoming an oval poleward boundary arc. The arcs then continue to move equatorward, and “lay down” along the auroral oval, leading to an explosive substorm onset (or expansion enhancement) that rapidly expands azimuthally with laying down of the arc, and later expands further with westward traveling surge sliding along the arc. This gives a longitudinally broad region of expansion-phase aurora, and a similarly broad magnetic depression, indicating large and broad substorm energy release. The surge sliding along the poleward boundary arc occurs at a higher latitude than the laydown portion of onset, so that sliding leads to later magnetic depressions at locations west of the initial onset and at higher latitudes. The polar cap arc appears to be pushed equatorward and westward (for By >0) by the flow of new plasma moving across the polar cap, the arc being like a weather front that demarcates an advancing boundary between plasma regimes along polar cap field lines. The onset and expansion of the activity appear to be driven by the flow of this new plasma from the polar cap to the region of expansion phase activity.