The enigmatic aurora borealis has kept scientists on a constant quest to understand the mysterious forces behind the spectacular dance of lights that brightens the North sky at night. Auroras can be observed near the Earth’s northern (aurora borealis) and southern (aurora australis) magnetic poles, in colorful green, blue or red displays.

The spectacular events have been linked to geomagnetic substorms, which are powered by explosions of magnetic energy one third of the way to the moon. This triggers the magnetic reconnection phenomenon, when magnetic field lines merge into new shapes, causing the sudden brightening and movement of the aurora borealis.

“We discovered what makes the Northern Lights dance,” said Dr. Vassilis Angelopoulos of the University of California, Los Angeles. Scientists have been trying for decades to find out what exactly triggers the burst of energy in Earth’s magnetosphere.

With the help of the NASA THEMIS satellites, short for Time History of Events and Macroscale Interactions during Substorms, researchers identified the magnetic explosions that produce dynamic changes in the auroral displays.

Uncovering when, where and how these substorms occur will give them a starting point for creating realistic substorm models that would help predict the intensity and effects of magnetic storms. Substorms are not only responsible for the impressive spectacle of light, but also for disrupting orbiting satellites and power grids.

David Sibeck, THEMIS project scientist at NASA’s Goddard Space Flight Center, explained that the magnetic reconnection releases the energy captured and stored by Earth’s magnetic field lines from the solar wind back toward the Earth’s atmosphere, which creates halos of shimmering auroras circling the northern and southern poles.

Prior to the observations from the THEMIS satellites, there have been two theories regarding the substorms. One theory placed substorms closer to Earth, one-sixth of the way to the moon, where large amounts of particles are sent toward our planet from solar wind currents' disruptions, causing the burst of light.

The second theory, confirmed by this study, places the substorms one third of the distance to the moon, where magnetic lines are stretched so far back that they snap into a new shape, similar to a rubber band when it is stretched too far, in a phenomenon known as magnetic reconnection.

With the help of the five THEMIS satellites and a network of 20 ground observatories located throughout Canada and Alaska, scientists have captured invaluable data about how and when these substorms occur.

On February 26, 2008, during an alignment of the five satellites (which takes place every four days along the equator), scientists captured the beginning of an isolated substorm in space, while at the same time, ground-based observatories recorded auroral brightening and space currents at ground level.

It was for the first time that scientists got a confirmation that the magnetic reconnection was indeed responsible for triggering substorms. According to a reconnection model of substorms, the phenomenon occurs following a particular pattern: the reconnection phenomenon is followed the auroral brightening, and the rapid expansion of the aurora towards the poles. The process ends with the redistribution of electrical currents flowing in space around Earth, scientists concluded.

The findings of the THEMIS project will appear in the July 24 issue of Science Express and the August 14 issue of the journal Science. THEMIS is the fifth medium-class mission under NASA’s Explorer Program, and provides investigation opportunities in heliophysics and astrophysics.