What Makes Northern Lights Dance

A collection of ground-based All-Sky Imagers (ASI) captures the aurora brightening caused by a substorm. This network is an important part of the THEMIS mission and is considered the sixth satellite. Video by NASA/Goddard Space Flight Center Scientific Visualization Studio
By Marc Kaufman
Washington Post Staff Writer
Friday, July 25, 2008

The mysterious sudden brightening and wavelike movements of the aurora borealis, also called the Northern Lights, are caused by periodic explosions of magnetic energy 80,000 miles above Earth, NASA researchers reported.

Using five NASA satellites designed to observe and record magnetic and electrical activity around Earth, the researchers found that the sudden changes were caused by magnetic reconnection, a common process that occurs throughout the universe when magnetic field lines stretched by the force of the solar wind suddenly snap into a new shape, like a rubber band that has been stretched too far.

"We discovered what makes the Northern Lights dance," said Vassilis Angelopoulos of the University of California at Los Angeles. Angelopoulos is the principal investigator for the satellite project, called THEMIS for Time History of Events and Macroscale Interactions during Substorms. The satellites, launched in February 2007, were designed to learn more about the auroras and other space weather phenomena that can interfere with orbiting telecommunications satellites and electric power grids on Earth, as well as endanger astronauts in space.

"As they capture and store energy from the solar wind, the Earth's magnetic field lines stretch far out into space," said David Sibeck, THEMIS project scientist at NASA's Goddard Space Flight Center in Greenbelt. "Magnetic reconnection releases the energy stored within these stretched magnetic field lines, flinging charged particles back toward the Earth's atmosphere."

The solar wind is a stream of charged particles that fly off the sun at speeds as great as 1 million mph. As solar wind blows past Earth, it deforms the planet's magnetic field, stretching it in the direction away from the sun.

The auroras occur regularly -- on average, two or three times a week in the skies around the North and South poles. They are different from the faint glows that tend to appear around the poles, which are themselves the result of particles in space coming down magnetic field lines. The auroras, instead, are the result of the dramatic substorms identified by THEMIS.

Since the beginning of the space age, scientists have debated whether auroras' dramatic movements were the result of magnetic short circuits relatively close to the Earth or to the reconnection events further out in space. The THEMIS observations answered that question, Angelopoulos said.

The five THEMIS satellites line up once every four days along the equator and make observations that are synchronized with 20 ground stations in Canada and Alaska. Through this process, researchers learned that the Northern Lights began shimmering soon after each significant "reconnection" event in the magnetosphere, which was then followed by a redistribution of the electrical currents that flow through space around Earth.

Solving the mystery of where, when and how substorms -- the localized expression of intense space weather -- occur could help scientists construct better models to predict a magnetic storm's intensity and effects. With more telecommunications, military and intelligence activity and terrestrial weather prediction based on clean data from satellites, knowing when problems may arise has become increasingly important, officials said.

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