Secret of Colorful Auroras Revealed
Artist's rendition of magnetic reconnection triggering substorm onset, as captured by NASA/THEMIS spacecraft.
Credit: Walt Feimer NASA/GSFC.

A flotilla of NASA probes has solved the 30-year mystery behind the most colorful aurora displays on Earth and the explosive magnetic "substorms" that spawn them.

NASA?s five THEMIS spacecraft in different orbits around Earth spotted the trigger for the substorms, powerful energy bursts in the planet?s magnetic field that can interfere with satellites, power grids and supercharge the aurora borealis, also known as the northern lights.

"We discovered what sparks the magnificent light show of the aurora," said THEMIS principal investigator Vassilis Angelopoulos, a space scientist at the University of California, Los Angeles.

Angelopoulos and his team used the THEMIS probes to monitor the energy levels in Earth?s magnetic field. In February of this year, the spacecraft spotted substorms originating in the tail of the magnetosphere that streams out away from the sun. As energy levels in the magnetic field lines built up, they drew ever closer to one another until they reconnected, setting off a storm, researchers said.

For the last three decades, researchers were undecided on whether the substorms stemmed from magnetic field lines reconnecting, or originated much closer to Earth where they were triggered by explosive instability.

"Our data show clearly and for the first time that magnetic reconnection is the trigger," said Angelopoulos.

The research is detailed in the July 24 online edition of the journal Science.

First discovered in the 19th century, magnetic substorms are recurring energy bursts stemming from the release of charged particles — collected from the sun?s solar wind — in the Earth?s magnetic field. The high-energy particles zoom down the Earth?s magnetic field lines until they collide with the planet?s upper atmosphere to create dazzling, shifting colors in the aurora borealis.

Researchers hope that by better understanding the storms, they will be able to prepare for or predict major space weather events before the cosmic tempests interfere with communications or endanger astronauts in Earth orbit.

"We need to understand this environment and eventually be able to predict when these large energy releases will happen so astronauts can go inside their spacecraft and we can turn off critical systems on satellites so they will not be damaged," Angelopoulos said, adding that previous missions could only track bursts at a single point and time. ?To resolve this question properly requires correlations and signal-timing at multiple locations. This is precisely what was missing until now."

NASA?s $200 million THEMIS spacecraft, short for Time History of Events and Macroscale Interactions During Substorms, launched in February 2007 on a two-year mission to hunt for the substorm source. The five dishwasher-sized spacecraft are arranged in different orbits between the Earth and moon, and are working in conjunction with 20 ground-based observatories across the northern United States and Canada to monitor substorm activity.