Choreographer of Northern Lights Revealed

Auroral oval (in false color) as seen from space. The red indicates the brightest aurora and blue the dimmest. CREDIT: Holzworth/Meng, NSSDC, NASA

The gracefully flitting symphony of lights that color polar skies have their very own high-energy choreographer.

Astronomers have long puzzled over the invisible forces at work that give rise to the auroras, or Northern Lights (at the South Pole they're called Southern Lights). New measurements from a group of satellites reveal that conditions at boundaries between layers of electrified particles control the lights.

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While the finding does expose some of the magic veiled by the sky lights, it has also added another layer of mystery, according to the scientists.

"As always the more we learn about it the more questions are raised," said lead researcher G?ran Marklund from the Alfven Laboratory, at the Royal Institute of Technology in Stockholm.

"So in a way one can say it's a never-ending story because you figure something out and then you realize that nature is even more complicated than you could ever guess," Marklund told SPACE.com.

Light show

Researchers knew that relatively static electric fields, which hover parallel to Earth's magnetic fields, play an important role in the acceleration of electrons that causes auroras to shine.

Inside these electric fields are invisible structures representing electric potentials. When charged particles streaming through space hit upon the structures, they get accelerated in opposite directions. In a celestial crash course, the charged particles ram into the ionosphere, a region in the upper atmosphere, where they transfer energy to the ionospheric molecules of oxygen and nitrogen. The result: glowing arcs and swirls.

The electric potential structures come in two flavors: symmetric U-shaped and asymmetric S-shaped. In 2004, Marklund discovered the U-shaped circuits form at a plasma boundary between a region within the magnetotail at equatorial latitudes and one at higher latitudes. The S-shapes occur at the boundary between the plasma sheet (at the inner edge of the auroral oval) and the polar cap.

Close contact

A group of four spacecraft, part of the European Space Agency's Cluster mission, orbiting in a triangular pyramid formation provided a behind-the-scenes look at an aurora's choreographer.

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.

So the shape morphing--concurrent with plasma and electric field conditions--strengthen Marklund's theory. "These results nicely demonstrate that the way the potential structure looks really reflects the plasma conditions and the electric current system," Marklund said.

He added, "We don't know yet how long [the potential structures] last or how they are distributed in terms of altitude."

The results are detailed in the Jan. 13 issue of the Journal of Geophysical Research.

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