An anonymous reader writes
"Recently four spacecraft, part of the European Space Agency's Cluster mission, provided a behind-the-scenes look at what choreographs the auroras or Northern Lights (at the South Pole they're called Southern Lights). Researchers have known that relative static electric fields, which hover parallel to Earth's magnetic fields, play an important role in the acceleration of electrons that causes the auroras to shine.
From the Space.com article:
In the recent study, one of the spacecraft crossed the auroral arc at high altitude in the Earth's magnetotail. As expected, it detected the U-shaped structure when crossing the boundary within the plasma sheet. Just 16 minutes later another Cluster spacecraft crossed the same boundary and revealed an asymmetric S-shaped structure, which was a surprise since the S-shape was thought to arise at the polar cap boundary.
Within that 16-minute period, the plasma density and associated electric currents plummeted at the plasma boundary. So the boundary ended up resembling the steep drop-off in particle density between the aurora edge and the polar cap."
This fits with the theory set forth in 2004 by Göran Marklund from the Alfvén Laboratory, that U-shaped circuits form at a plasma boundary between a region within the magnetotail at equatorial latitudes and one at higher latitudes and the S-shapes occur at the boundary between the plasma sheet (at the inner edge of the auroral oval) and the polar cap."
