Left side. Auroral oval (in false colour) as seen from space, overlaid on top of a visible image of Earth. The red indicates the brightest aurora and blue the dimmest. The brightest aurora is found at midnight.
Right side. Locations of 4 auroral oval crossings by the Cluster spacecraft projecting down to the ionosphere with respect to nightside auroral plasma boundaries. In this projection the Earth is viewed from above the North pole and polar cap, the Sun is located at the top of the image. Events 1 and 2 from early 2001 were both located at the boundary between the Central Plasma Sheet (CPS, in green) and the Plasma Sheet Boundary Layer (PSBL, in blue). Events 3 and 4 in 2002 were found at the boundary between the PSBL and the polar cap.