Dec. 11, 2007: NASA's fleet of THEMIS spacecraft, launched less than 8 months ago, has made three important discoveries about spectacular eruptions of Northern Lights called "substorms" and the source of their power. The discoveries include giant magnetic ropes that connect Earth's upper atmosphere to the Sun and explosions in the outskirts of Earth's magnetic field.
"The mission is only beginning but THEMIS is already surprising us," says Vassilis Angelopoulos the mission's principal investigator at the University of California, Los Angeles.
The discoveries began in March less than a month after the five THEMIS satellites had been activated. "On March 23, 2007, a substorm erupted over Alaska and Canada producing vivid auroras for more than two hours." A network of ground cameras organized to support THEMIS photographed the display from below while the satellites measured particles and fields from above.
Right: Auroras over Alaska on March 23-24, 2007. Photo credit: Daryl Pederson. [More]
Right away the substorm surprised investigators: "The auroras surged westward twice as fast as anyone thought possible, crossing 15 degrees of longitude in less than one minute," says Angelopoulos. The storm had traversed an entire polar time zone in 60 seconds flat!
Also, "the display was surprisingly bursty." Photographs taken by ground cameras and NASA's Polar satellite (also supporting the THEMIS mission) revealed a series of staccato outbursts each lasting 10 minutes or so. "Some of the bursts died out while others reinforced each other and went on to become major events."
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Scientists have been tracking and studying substorms for more than a century, yet these phenomena remained mostly unknown until THEMIS went into action.
Even more impressive was the substorm's power. Angelopoulos estimates the total energy of the two-hour event at five hundred thousand billion (5 x 1014) Joules. That's approximately equivalent to the energy of a magnitude 5.5 earthquake.
Where does all that energy come from? THEMIS may have found an answer:
"The satellites have found evidence for magnetic ropes connecting Earth's upper atmosphere directly to the Sun," says Dave Sibeck, project scientist for the mission at the Goddard Space Flight Center. "We believe that solar wind particles flow in along these ropes, providing energy for geomagnetic storms and auroras."
A "magnetic rope" is a twisted bundle of magnetic fields organized much like the twisted hemp of a mariner's rope. Spacecraft have detected hints of these ropes before, but a single spacecraft is insufficient to map their 3D structure. THEMIS's five satellites were able to perform the feat.