Two-component auroras

By examining Polar Ultraviolet Imager auroral images at the 160-180 nm N₂ Lyman-Birge-Hopfield emission band, we find that the auroral dynamics associated with substorms shows a two-component feature. An auroral feature, which is called the two-cell aurora, is associated with the increased magnetospheric convection in response to a southward interplanetary magnetic field. A substorm expansion may then occur, adding an additional aurora in the midnight sector. As the substorm continuously increases its strength, the substorm aurora connects with the two-cell aurora, forming a well-defined crescent-shape aurora. It is also found that the occurrence rate of the two-cell aurora depends on universal time.